Data line disturbance free memory block divided flash memory and microcomputer having flash memory therein

ABSTRACT

A semiconductor device having an electrically erasable and programmable nonvolatile memory, for example, a rewritable nonvolatile memory including memory cells arranged in rows and columns and disposed to facilitate both flash erasure as well as selective erasure of individual units of plural memory cells. The semiconductor device which functions as a microcomputer chip also has a processing unit and includes an input terminal for receiving an operation mode signal for switching the microcomputer between a first operation mode in which the flash memory is rewritten under control of a processing unit and a second operation mode in which the flash memory is rewritten under control of separate writing circuit externally connectable to the microcomputer.

[0001] This application is a continuation of U.S. application Ser. No.09/987,957, filed Nov. 16, 2001, which, in turn, is a continuation ofU.S. application Ser. No. 09/705,835, filed Nov. 6, 2000, now U.S. Pat.No. 6,335,879; which, in turn, was a continuation of application Ser.No. 09/414,944, filed Oct. 8, 1999, now U.S. Pat. No. 6,166,953; which,in turn, was a continuation of application Ser. No. 09/144,194, filedAug. 31, 1998, now U.S. Pat. No. 6,064,593; which, in turn, was acontinuation of application Ser. No. 08/788,198, filed Jan. 24, 1997,now U.S. Pat. No. 6,026,020; which, in turn, was a continuation ofapplication Ser. No. 08/473,114, filed Jun. 7, 1995, now U.S. Pat. No.5,768,194; and which, in turn, was a continuation of application Ser.No. 08/031,877, filed Mar. 16, 1993, now abandoned; and the entiredisclosures of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a nonvolatile flash memory inwhich information is rewritable by electrical erasing/writing and amicrocomputer incorporating the same.

[0003] JP-A-1-161469 (Laid-open on Jun. 26, 1989) describes amicrocomputer having, as a programmable nonvolatile memory, an EPROM(erasable and programmable read only memory) or an EEPROM (electricallyerasable and programmable read only memory) carried on a singlesemiconductor chip. Data and programs are held in such an on-chipnonvolatile memory of the microcomputer. Since information stored in theEPROM is erased by means of ultraviolet rays, the EPROM must be removedfrom a system on which it is mounted in order for the EPROM to berewritten. The EEPROM can be erased and written electrically andtherefore information stored therein can be rewritten with the EEPROMmounted on a system. However, memory cells constituting the EEPROM mustbe comprised of, in addition to memory devices such as MNOSs (metalnitride oxide semiconductors), selecting transistors and hence theEEPROM requires a relatively large chip occupation area being, forexample, about 2.5 to 5 times as large as that of the EPROM.

[0004] JP-A-2-289997 (Laid-open on Nov. 29, 1990) describes asimultaneous erasing type EEPROM. This simultaneous erasing type EEPROMcan be described as operating as a flash memory, such as described inthe present specification. In the flash memory, information can berewritten by electrical erasing and writing, each memory cell can beconstructed of a single transistor as in the EPROM and, functionally,all memory cells or a block of memory cells can be erased simultaneouslyby electrical erasing. Accordingly, in the flash memory, informationstored therein can be rewritten with the flash memory mounted on asystem, the time for rewrite can be shortened by virtue of itssimultaneous erasing function and contribution to reduction of the areaoccupied by a chip can be accomplished.

[0005] U.S. Pat. No. 5,065,364 (issued on Nov. 12, 1991) shows a flashmemory of the type in which an array of electrically erasable andrewritable memory cells having control gates, drains and sources isdivided into a plurality of memory blocks in a unit of data line, sourcelines in common to each block are led out and a voltage complying withan operation is applied separately to a source line by means of a sourceswitch provided in each source line. At that time, ground potential isapplied to the source line of a block selected for writing. A voltageVDI of, for example, 3.5V is applied to the source line of a block notselected for writing. The voltage VDI guards against word linedisturbance. The word line disturbance referred to herein is aphenomenon that for example, in a memory cell having a word lineconditioned for selection and a data line conditioned for unselection,the potential difference between the control gate and floating gate isincreased and as a result, electric charge is discharged from floatinggate to control gate to decrease the threshold of the memory celltransistor.

[0006] JP-A-59-29488 (laid-open on Feb. 16, 1991) and JP-A-3-78195(laid-open on Apr. 3, 1991) describe an ultraviolet light-erasable EPROMin which sources of memory cells connected with the same word line areconnected in common and a source potential control switch is providedfor the commonly connected sources. JP-A-3-78195 (laid-open on Apr. 3,1991) describes an ultraviolet light-erasable EPROM in which sources ofmemory cells connected with adjacent two word lines are connected incommon and a source potential control switch is provided for eachadjacent two word lines. Each of the inventions disclosed in these threereferences is intended to provide a solution to a problem of erroneouswriting/reading caused by leak current from an unselected memory cell inan EEPROM.

[0007] U.S. application Ser. No. 07/942,028 filed Sep. 8, 1992, which isa continuation application of U.S. application Ser. No. 07/568,071 filedAug. 16, 1990, discloses a structure of a flash memory in which sourcesof memory cells are connected in common for the purpose of preventing aword line disturb problem for a writing operation.

[0008] Meanwhile, JP-A-3-14272 (laid-open on Jan. 22, 1991),JP-A-3-250495 (laid-open on Nov. 8, 1991) and JP-A-2-241060 (laid-openon Sep. 25, 1990) describe division of a memory cell array in a unit todata line.

SUMMARY OF THE INVENTION

[0009] The present inventors have first studied the fact that a flashmemory is carried on a microcomputer to find out the following points.

[0010] (1) Programs and data are stored in a ROM incorporated or builtin the microcomputer. Data is classified into data of a large capacityand data of a small capacity. When the programs and data are to berewritten, the former data is typically rewritten in a large unit ofseverals of tens of KB (kilobyte) and the latter data is typicallyrewritten in a small unit of severals of tens of B (byte). At that time,if the flash memory is erased in a unit of chip batch or in a unit ofmemory block of the same size, inconvenience that the erase unit matcheswith a program area but is excessively large for a data area to impairease of use thereof may occur or the converse case may occur.

[0011] (2) When part of information held in the flash memory is desiredto be rewritten after the microcomputer is mounted on a system, itsuffices to use part of the memory block holding the information ofinterest as an object to be rewritten. But if all simultaneouslyerasable memory blocks have an equal storage capacity, then even whenrewrite of only a smaller amount of information than the storagecapacity of a memory block is desired, the memory block of a relativelylarge storage capacity must be erased simultaneously and thereafterwrite is carried out over the whole of the memory block in question,with the result that time is consumed wastefully for rewrite ofinformation not substantially required to be rewritten.

[0012] (3) Information to be written into the flash memory is determinedin accordance with the system to which the microcomputer is applied butefficiency may sometimes be degraded when the information is all writtenfrom the beginning with the microcomputer of interest mounted on thesystem.

[0013] (4) When the flash memory is rewritten with the microcomputermounted, it sometimes suffices that only part of information of a memoryblock, standing for an object to be rewritten, is rewritten. But even inthis case, if information to be written into the whole of the memoryblock which has been erased simultaneously is all received sequentiallyexternally of the microcomputer and rewritten, all of the information tobe written into the whole of the memory block of interest will have tobe received from the outside in spite of the fact that it suffices torewrite only part of information of the memory block to be rewritten,and transfer, from the outside, of information not substantiallyrequired to be rewritten, that is, information held internally inadvance of rewrite must be repeated, resulting in wastefulness oftransfer of information for partial rewrite of the memory block.

[0014] (5) Because of information storing mechanism, the time forrewriting the flash memory through simultaneous erasing is far longer ascompared to a memory such as RAM (random access memory) and so the flashmemory cannot be rewritten on real time base in synchronism with machinecontrol operation by the microcomputer.

[0015] The present inventors have studied the division of memory blocksin a unit of data line to find that the size of the minimum memory blockcan be decreased more easily by division into memory blocks in a unit ofword line and using sources in common in a block and this isadvantageous also from the standpoint of improving ease of use of theflash memory built in the microcomputer as studied firstly. When thedivision into memory blocks in a unit of data line is employed, allmemory cells of a selected block for writing arranged in line and havingdrains connected to a data line applied with a write high voltage sufferfrom data line disturbance. The data line disturbance is a phenomenonthat for example, in a memory cell associated with a word line notrendered to be selected and a data line rendered to be selected forwriting, an electric field between the source and drain is increased, sothat hot holes are injected from drain to floating gate to decrease thethreshold of the memory cell transistor.

[0016] A major object of the invention is to provide a microcomputerincorporating a flash memory which is easy to use. More particularly, afirst object of the invention is to provide a microcomputer capable ofmaking highly efficient a processing of initial write of informationinto the built-in flash memory. A second object of the invention is toimprove rewrite efficiency of part of information held in some of memoryblocks of the flash memory by eliminating wastefulness of writeoperation after simultaneous erasing of the memory blocks of interest. Athird object of the invention is to improve rewrite efficiency byeliminating wasteful transfer operation, from the outside, of writeinformation necessary for partial rewrite of a memory block. A fourthobject of the invention is to change information held in the flashmemory on real time base in synchronism with control operation by themicrocomputer.

[0017] Further, the invention has for its object to provide a flashmemory in which the minimum size of memory block obtained by usingsources in common in an electrically rewritable nonvolatile memorydevice can be decreased. Still another object is to prevent an erroneousoperation due to data line disturbance in a unselected memory block forwriting from occurring when formation of memory blocks is effected in aunit of word line.

[0018] Major aspects of the present invention will now be described.

[0019] More specifically, a microcomputer comprises, on a singlesemiconductor chip, a central processing unit and a nonvolatile flashmemory in which information to be processed by the central processingunit is rewritable by electrical erase and write, and the microcomputeris provided with an input terminal of an operation mode signal fordesignating a first operation mode in which rewrite of the flash memoryis controlled by a circuit built in the semiconductor chip and a secondoperation mode in which it is controlled by a unit provided externallyof the semiconductor chip.

[0020] When the central processing unit performs rewrite control inaccordance with designation of the first operation mode, a rewritecontrol program to be executed by the central processing unit may beheld in a mask ROM or a rewrite control program precedently stored inthe flash memory may be transferred to a RAM and executed.

[0021] The fact that the amount of information to be stored in the flashmemory in accordance with an application differs in accordance with thekind of the information such as for example a program, a data table orcontrol data is taken into consideration. Then, in order that uponrewrite of part of information held in some of memory blocks of theflash memory, efficiency of rewrite can be improved by eliminatingwastefulness of write operation after simultaneous erasing of the memoryblocks of interest, a plurality of memory blocks having mutuallydifferent storage capacities may be allotted each for a simultaneouslyerasable unit in the flash memory.

[0022] When rewrite of the flash memory is controlled internally andexternally of the microcomputer, in order for a memory block to beerased simultaneously can be designated easily, a register in whichinformation for designating the memory block to be erased simultaneouslyis rewritably held may be incorporated in the flash memory.

[0023] When the built-in flash memory has, as a simultaneous erase unit,a plurality of memory blocks having mutually different storagecapacities, in order that the built-in RAM can be utilized as a workingarea or a data buffer area for rewrite of memory block, a memory blockhaving a storage capacity set to be smaller than that of the built-inRAM may be provided. In this case, for the sake of improving efficiencyof rewrite by eliminating wastefulness of transfer operation, from theoutside, of write information necessary for partial rewrite of thememory block, information held in the memory block having a storagecapacity smaller than that of the built-in RAM may be transferred to thebuilt-in RAM, all or part of the transferred information may be renewedon the RAM and the memory block of interest may be rewritten withrenewed information. Further, upon tuning of data such as control dataheld in the flash memory, in order that information held in the flashmemory can be changed on real time base in synchronism with controloperation by the microcomputer, a processing may be effected whereinaddresses of a specified area of the built-in RAM are controllablychanged and arranged so as to overlap addresses of the memory blockhaving the smaller storage capacity than the built-in RAM, that is,changed and arranged so that the overlapped RAM may be accessed when thememory block is accessed and after working has been done at thespecified address, the arranged address of the RAM is restored to theoriginal state and the contents of the memory block is rewritten withthe information at the specified address of the RAM.

[0024] In order to decrease the minimum block size more easily ascompared to the case where memory blocks are formed in a unit of dataline, memory blocks are defined by connecting a common source line tomemory cells having their control gates coupled to a single or aplurality of word lines in a unit of word line.

[0025] At that time, to take care of data line disturbance in anunselected memory block for writing, voltage output means is adoptedwhich can control, in a unit of memory block, potential of the sourceline to first potential and to second potential of higher level thanthat of the first potential upon write operation, whereby the voltageoutput means applies the first potential to a source line of a memoryblock including memory cells having an associated data line and anassociated word line applied with predetermined voltages so as to beselected for writing and applies the second potential to a source lineof a memory block including memory cells having an associated data lineapplied with the predetermined voltage and an associated word line notapplied with the predetermined voltage so as not to be selected forwriting.

[0026] In order to improve ease of use in the formation of memory blocksin a unit of word line, a plurality of memory blocks include a single ora plurality of large memory blocks associated with a relatively largenumber of word lines and a single or a plurality of small memory blocksassociated with a relatively small number of word lines.

[0027] At that time, in order to minimize the data line disturbancetime, the large memory block and the small memory block have data linesin common and arranged separately in line, a selection circuit forselecting a data line upon write and read operations is arranged nearthe large memory block, a transfer gate circuit is inserted in datalines which are associated in common with the large memory block andsmall memory block, and a control circuit is provided which cuts off thetransfer gate circuit upon write of the large memory block.

[0028] According to the above-mentioned aspects of the invention, wheninformation is initially written into the flash memory in the phasepreceding mounting of the microcomputer according to the invention, theinformation can be written efficiently under the control of the externalwrite device such as a PROM writer by designating the second operationmode.

[0029] For example, programs, data tables or control data are writteninto the plurality of memory blocks having mutually different capacitiesand defined each as a simultaneously erasable unit, in accordance with astorage capacity of each memory block.

[0030] When the microcomputer is mounted on the system and thereafterthe flash memory is rewritten, the first operation mode is designated tocause, for example, the central processing unit built in themicrocomputer to execute control of rewrite. In this case, data of arelatively large information amount can be written in a memory block ofa relatively large storage capacity and data of a relatively smallinformation amount can be written in a memory block of a relativelysmall storage capacity. Namely, a memory block having a storage capacitymeeting the information amount to be stored can be utilized.Accordingly, even when a given memory block is erased simultaneously forrewrite of part of information held in the flash memory, suchwastefulness that an information group substantially not required to berewritten is erased concurrently and thereafter written again can beprevented as far as possible.

[0031] Especially, when of the plurality of memory blocks, a memoryblock having a storage capacity set to be smaller than that of thebuilt-in RAM is provided, this memory block may be utilized as a workarea or a data buffer area for rewrite of memory block. Moreparticularly, when the flash memory is rewritten with the microcomputermounted, information in a memory block to be rewritten is transferred tothe built-in RAM, only partial information to be rewritten is receivedfrom the outside and rewritten on the RAM and then the flash memory isrewritten, whereby transfer, from the outside, of information heldinternally in advance of rewrite and not required to be rewritten neednot be repeated, so that wastefulness of information transfer forpartial rewrite of the memory block can be eliminated. Further, in theflash memory, the time for simultaneously erasing a small memory blockis not so short that the flash memory per se can be rewritten on realtime base in synchronism with control operation by the microcomputer.But, by utilizing the built-in RAM as a work area or a data buffer areafor rewrite of memory block, the same data as that rewritten on realtime base can eventually be obtained in the memory block.

[0032] When memory blocks are defined each in a unit of word line, theminimum memory block has a storage capacity which corresponds to that ofone word line, regardless of the number of parallel input/output bits.Contrary to this, when memory blocks are defined each in a unit of dataline, the minimum memory block has a storage capacity corresponding tothe number of data lines which in turn corresponds to the number ofparallel input/output bits. This signifies that the storage capacity ofthe minimum memory block can be reduced more easily when memory blocksare defined in a unit of word line and especially in the case of amemory incorporated in the microcomputer wherein input/output of data iscarried out in a unit of byte or word, the minimum size of memory blockcan be reduced drastically. This contributes to further improvement inease of use of the flash memory built in the microcomputer andconsequently improvement in efficiency of rewrite of small scale data ina unit of memory block.

[0033] In a region near the source side end of the drain of anonvolatile memory device, electron and hole pairs are generated owingto a tunnel phenomenon between bands. In this case, when a relativelylarge electric field is generated between the source and drain, holes ofthe electron and hole pairs are accelerated by the electric field toturn into hot holes. The hot holes are injected to the floating gatethrough a tunnel insulating film. This state is referred to as data linedisturbance and when the data line disturbance affects the device for along time, the threshold of the memory device is decreased and thereresults an undesirable change of stored information which leads to anerroneous operation (data line disturbance fault). In an unselectedblock for writing, by applying second potential such as data linedisturbance prevention voltage to a source line of a memory cell toraise source potential, an electric field between the drain and sourceis weakened, thereby ensuring that holes of electron and hole pairsgenerated near the drain can be prevented from turning into hot holes toprevent a decrease in the threshold of memory transistor.

[0034] For prevention of the data line disturbance fault, minimizationof the data line disturbance time (the time for exposure to the dataline disturbance state) is effective. In this case, the data linedisturbance time affecting a small memory block owing to writeconcomitant with rewrite of a memory block having a large storagecapacity is relatively increased in comparison with the converse case.In view of this fact, by adopting an arrangement in which with respectto an intervening transfer gate circuit, memory blocks on the side of aY selection circuit are formed of large memory blocks and memory blockson the opposite side are formed of small memory blocks, the data linedisturbance time affecting memory cells of the memory blocks relativelynear the Y selection circuit owing to write of the memory blockrelatively remote from the Y selection circuit can be reduceddrastically as compared to the case of the converse arrangement of largememory blocks and small memory blocks. By virtue of this arrangementrelation between the large memory blocks and small memory blocks,erroneous operation due to data line disturbance can further besuppressed.

[0035] According to still another aspect of the present invention, thereis provided a microcomputer comprising a central processing unit, anelectrically rewritable flash memory, flash memory rewriting I/O portmeans capable of being coupled to a ROM writer for rewriting the flashmemory, switch means located between the central processing unit and theflash memory, and a rewriting mode decision means responsive to anexternally supplied operation mode signal for controlling the switchmeans and the flash memory rewriting I/O port means, the centralprocessing unit, the flash memory, the flash memory I/O port means, theswitching means and the rewriting mode decision means being formed in asingle semiconductor chip.

[0036] According to still another aspect of the present invention, thereis provided there is provided an electrically rewritable flash memorydevice comprising:

[0037] a memory cell array including a plurality of memory cellsarranged in rows and columns, each of the memory cells including anon-volatile memory element having first and second semiconductorregions formed in a first surface portion of a semiconductor substrate,a floating gate formed over and insulated from a second surface portionof the semiconductor substrate between the the first and secondsemiconductor regions, and a control gate formed over and insulated fromthe floating gate;

[0038] a plurality of first conductors extending in parallel with oneanother in a row direction over the semiconductor substrate, controlgates of memory cells in one row being connected in common to one firstconductor;

[0039] a plurality of second conductors extending in parallel with oneanother in a column direction over the semiconductor substrate, firstsemiconductor regions of memory cells in one column being connected incommon to one second conductor;

[0040] a plurality of common conductors extending in the row directionover the semiconductor substrate, second semiconductor regions of atleast two rows of memory cells being connected in common to one commonconductor such that the at least one row of memory cells having theirsecond semiconductor regions connected in common to one common conductorform a memory block, memory blocks so formed having different memorycapacities;

[0041] a plurality of common voltage control circuits formed in thesubstrate, one provided for each of the memory blocks, for generating acommon voltage assuming at least first and second voltage values; and

[0042] a control circuit formed in the substrate for generating acontrol signal indicating which of the memory blocks is subjected to anerasing/writing operation, the control signal being supplied to theplurality of common voltage control circuits so that individual commonvoltage control circuits apply to their associated common conductorscommon voltages each having one of the first and second voltage valuesdepending on the control signal to effect a writing operation with acommon voltage of the second voltage value applied to a common conductorfor a memory block which does not contain a memory cell selected for thewriting operation and to effect a simultaneous erasing operation with acommon voltage of the first voltage value applied to a common conductorfor a memory block selected for a simultaneous erasing operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a block diagram showing an embodiment of a microcomputeradopting an all over flash memory.

[0044]FIG. 2 is a block diagram showing an embodiment of a microcomputeradopting a flash memory along with a mask ROM.

[0045]FIG. 3 is a block diagram drawn from the viewpoint of rewrite of aflash memory by means of a general purpose PROM writer.

[0046]FIG. 4 is a block diagram drawn from the viewpoint of rewrite of aflash memory based on CPU control.

[0047]FIG. 5 is a memory map showing an example of a microcomputerapplied with an all over flash memory.

[0048]FIG. 6 is a memory map showing an example of a microcomputerhaving a flash memory along with a mask ROM.

[0049]FIG. 7 is a diagram for explaining an example of the schematiccontrol procedure of erase.

[0050]FIG. 8 is a diagram for explaining an example of the schematiccontrol procedure of write.

[0051]FIG. 9 is a diagram for explaining an example of an expedient toachieve rewrite of the flash memory on realtime base.

[0052]FIG. 10 is a diagram for explaining an example of a manner ofmaking partial rewrite of a memory block of the flash memory efficient.

[0053]FIGS. 11A and 11B are diagrams for explaining the principle of theflash memory.

[0054]FIG. 12 is a diagram for explaining the principle of constructionof a memory cell array using memory transistors of FIGS. 11A and 11B.

[0055]FIG. 13 is a circuit block diagram showing an example of a flashmemory in which a plurality of memory blocks are defined each in a unitof data line and having different storage capacities.

[0056]FIG. 14 is a block diagram showing an embodiment of furtherdetails of a microcomputer corresponding to the microcomputer of FIG. 1.

[0057]FIG. 15 is a plan view showing the packaged state of themicrocomputer of FIG. 14.

[0058]FIG. 16 is a block diagram showing the whole of the flash memoryincorporated in the microcomputer of FIG. 14.

[0059]FIG. 17 is a diagram for explaining an example of division intomemory blocks.

[0060]FIG. 18 is a diagram for explaining an example of a controlregister.

[0061]FIG. 19 is a timing chart showing an example of a memory readoperation in the flash memory.

[0062]FIG. 20 is a timing chart showing an example of a memory writeoperation in the flash memory.

[0063]FIG. 21 is a flow chart showing an example of details of the writecontrol procedure.

[0064]FIG. 22 is a flow chart showing an example of details of the erasecontrol procedure.

[0065]FIG. 23 is a diagram for explaining another example of divisioninto memory blocks.

[0066]FIG. 24 is a memory array portion configuration diagram showing anexample of a flash memory in which a plurality of memory blocks aredefined each in a unit of word line, having different capacities.

[0067]FIGS. 25A and 25B are diagrams for explaining an advantage of theembodiment shown in FIG. 24.

[0068]FIGS. 26A and 26B are diagrams for explaining an example ofvoltage conditions for countermeasures against data line disturbance inan unselected block for writing.

[0069]FIGS. 27A and 27B are diagrams for explaining the principle ofgeneration of data line disturbance and countermeasures there against.

[0070]FIG. 28 is a diagram for explaining the change of threshold of amemory cell with respect to data line disturbance time.

[0071]FIG. 29 is a circuit diagram for explaining the correlation ofdata line disturbance time between memory blocks of small storagecapacities and memory blocks of large storage capacities.

[0072]FIGS. 30A and 30B are diagrams showing an embodiment of a memoryarray in which a transfer gate circuit for selectively separating datalines is interposed between memory blocks.

[0073]FIG. 31 in an explanatory diagram in which an example of voltageconditions for countermeasures against data line disturbance is summedup.

[0074]FIG. 32 is a circuit diagram showing an example wherein a dummyword line is arranged between a memory block and a transfer gate.

[0075]FIG. 33 is a circuit diagram showing another example wherein adummy word line is arranged between a memory block and a transfer gatecircuit.

[0076]FIG. 34 is a circuit diagram showing still another example whereina dummy word line is arranged between a memory block and a transfer gatecircuit.

[0077]FIG. 35 is a diagram for explaining a memory array in which twomemory blocks are arranged on each side of a transfer gate circuit.

[0078]FIG. 36 is a circuit diagram showing an example of a memory arrayin which the number of word lines of simultaneously erasable memoryblocks is increased sequentially.

[0079]FIG. 37 is a diagram for explaining an example of a memory arrayin which a transfer gate circuit is arranged between a group of largememory blocks and a group of small memory blocks.

[0080]FIG. 38 is a circuit diagram showing an example of a memory arrayin which the data line structure is constructed of main data lines andsubsidiary data lines.

[0081]FIG. 39 is a diagram for explaining an example in whichsimultaneously erasable memory blocks are arranged on the left and rightsides of an X address decoder.

[0082]FIG. 40 is a diagram for explaining an example of a controlcircuit in FIG. 39.

[0083]FIG. 41 is a diagram for explaining an embodiment in whichredundant words are provided in a memory block.

[0084]FIG. 42 is a diagram for explaining an embodiment in which memoryblocks dedicated to redundancy are provided.

[0085]FIG. 43 is a diagram for explaining an embodiment in which somememory blocks are formed into one-time programmable areas.

[0086]FIG. 44 is a diagram for explaining an embodiment in which somememory blocks are formed into mask ROM's.

[0087]FIG. 45 is a diagram for explaining an example of a layout patternof memory blocks.

[0088]FIG. 46 is a diagram for explaining a layout pattern in whichtransfer gate MOS transistors are provided between memory blocks.

[0089]FIG. 47 is a diagram for explaining a pattern in which the drainof a dummy cell is floating in contrast to the configuration of FIG. 46.

[0090]FIG. 48 is a diagram for explaining a layout pattern in whichtransfer MOS transistors are substantially increased in size.

[0091]FIG. 49 is a block diagram showing an embodiment of the whole of aflash memory applied with pluralization of memory blocks in a unit ofword line and countermeasures against data line disturbance.

[0092]FIG. 50 is a block diagram showing details of a control circuitincluded in the flash memory of FIG. 49.

[0093]FIG. 51 is a diagram for explaining details of a power supplycircuit included in the flash memory of FIG. 49.

[0094]FIG. 52 is a waveform diagram showing output voltages generatedfrom the power supply circuit of FIG. 51.

[0095]FIGS. 53A and 53B are diagrams for explaining details of an Xaddress decoder included in the flash memory of FIG. 49.

[0096]FIG. 54 is a diagram for explaining details of an example of anerase circuit included in the flash memory of FIG. 49.

[0097]FIG. 55 is an operational timing chart of the erase circuit ofFIG. 54.

[0098]FIG. 56 is a timing chart of a series of operations related toerase in the flash memory shown in FIG. 49.

[0099]FIG. 57 is a timing chart of a series of operations related towrite in the flash memory shown in FIG. 49.

[0100]FIGS. 58A to 58I are longitudinal sectional views of a device atvarious stages of the production processes of various transistors forconstituting the flash memory or the microcomputer incorporating thesame.

[0101]FIG. 59 is a diagram for explaining techniques of erasing theflash memory.

[0102]FIG. 60 is a longitudinal sectional view for explaining asemiconductor substrate/well structure corresponding to sector erase.

[0103]FIG. 61 is a longitudinal sectional view for explaining anothersemiconductor substrate/well structure.

[0104]FIG. 62 is a longitudinal sectional view for explaining stillanother semiconductor substrate/well structure corresponding to sectorerase.

[0105]FIG. 63 is a diagram illustrating an overlap of a specifiedaddress area of the random access memory with a predetermined addressarea of the flash memory.

[0106]FIG. 64 is a diagram showing an example of a RAM control register.

[0107]FIG. 65 is a diagram illustrating an address setting for aspecified address area of the random access memory.

[0108]FIG. 66 is a diagram showing an example of a chip selectcontroller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0109] Embodiments of the present invention will be described insequence of the following items.

[0110] [1] A microcomputer adopting an allover flash memory

[0111] [2] A microcomputer adopting a mask ROM and a flash memory

[0112] [3] write of information by means of a general purpose PROMwriter

[0113] [4] A write control program under the control of a CPU

[0114] [5] Proper use of write by the general purpose PROM writer orwrite by the CPU control

[0115] [6] Expedient to achieve rewrite on real time base

[0116] [7] Making partial rewrite of a memory block efficient

[0117] [8] The principle of the flash memory

[0118] [9] Formation of a plurality of memory blocks having differentstorage capacities and defined each in a unit of data line

[0119] [10] Details of a microcomputer corresponding to FIG. 1

[0120] [11] A control circuit for rewrite of flash memory FMRY

[0121] [12] Details of the rewrite control procedure of the flash memoryFMRY

[0122] [13] Formation of a plurality of memory blocks having differentstorage capacities and defined each in a unit of word line

[0123] [14] Countermeasures against data line disturbance in anunselected block for writing

[0124] [15] Correlation of data line disturbance time between memoryblocks

[0125] [16] A transfer gate circuit for data line separation

[0126] [17] Dummy word lines

[0127] [18] Various forms of pluralization of memory blocks in a unit ofword line

[0128] [19] Layout configuration of memory blocks

[0129] [20] The whole of a flash memory applied with countermeasurementsagainst data line disturbance

[0130] [21] A method for production of a flash memory

[0131] [22] A semiconductor substrate/well structure meeting sectorerase

[0132] [1] A Microcomputer Adopting an Allover Flash Memory

[0133]FIG. 1 is a block diagram showing an embodiment of a microcomputeradopting an allover flash memory (the whole of the ROM in themicrocomputer being in the form of a flash memory). In a microcomputerMCU shown in the figure, a central processing unit CPU, a non-volatileflash memory FMRY in which information to be processed by the centralprocessing unit CPU is rewritable by electrical erase and write,peripheral circuits such as timer TMR, serial communication interfaceSCI, random access memory RAM and input/output circuit I/O, and acontrol circuit CONT are formed on a single semiconductor chip CHP suchas silicon through known semiconductor integrated circuit productiontechniques. In the flash memory FMRY, information is rewritable byelectrical erase and write and like an EPROM, its memory cell can beconstructed of a single transistor; and besides it has the function ofelectrically erasing all memory cells simultaneously or a block ofmemory cells (a memory block) simultaneously. The flash memory FMRY hasa plurality of memory blocks each defined as a simultaneously erasableunit. In FIG. 1, LMB designates a large memory block having a relativelylarge storage capacity and SMB represents a small memory block having arelatively small storage capacity. The storage memory of the smallmemory block SMB is designed to be not larger than that of the randomaccess memory RAM. Accordingly, the random access memory RAM can receivedata transfer from the small memory block SMB to hold the informationtemporarily and can be used as a work area or data buffer area forrewriting. Requisite data and programs are written in the flash memoryFMRY. Details of the flash memory FMRY will be described later.

[0134] The flash memory FMRY is allowed to rewrite its storageinformation under the control of the central processing unit CPU orunder the control of a unit externally of the semiconductor chip CHPsuch as a general purpose PROM writer while the microcomputer MCU ismounted on a system. In the figure, MODE denotes an operation modesignal for selectively designating a first operation mode which causesthe central processing unit to control rewrite of the flash memory FMRYand a second operation mode which causes the external unit to controlrewrite of the flash memory FRMY and the operation mode signal isapplied to a mode signal input terminal P mode on the semiconductor chipCHP.

[0135] [2] A Microcomputer Adopting a Mask ROM and a Flash Memory

[0136]FIG. 2 is a block diagram showing an embodiment of a microcomputeradopting a mask ROM along with a flash memory. In microcomputer MCUshown in the figure, part of the flash memory FMRY of FIG. 1 is replacedwith a mask read only memory MASKROM. Held in the mask read only memoryMASKROM are data and programs which need not be rewritten. A flashmemory FMRY shown in FIG. 2 has a plurality of small memory blocks SMBeach defined as a simultaneously erasable unit.

[0137] [3] Write of Information by Means of a General Purpose PROMWriter

[0138]FIG. 3 is a block diagram drawn from the viewpoint of rewrite of aflash memory FMRY by means of a general purpose PROM writer. In thefigure, as an example of the mode signal MODE, MD0, MD1 and MD2 areindicated. The mode signals MD0 to MD2 are supplied to a control circuitCONT. A decoder included in the control circuit CONT, though notlimitedly, decodes the mode signals MD0 to MD2 and decides whether anoperation mode which does not require write into the flash memory FMRYis designated or whether the first operation mode or the secondoperation mode is designated. At that time, if designation of the secondoperation mode is determined, the control circuit CONT performs controlsuch that it designates I/O ports which are to interface with thegeneral purpose PROM writer PRW and permits the external general purposePROM writer PRW to directly access the built-in flash memory FMRY. Morespecifically, an I/O port PORTdata for performing input/output of databetween the writer and the flash memory FMRY, an I/O port PORTaddr forsupplying address signals to the flash memory FMRY and an I/O portPORTcont for supplying various kinds of control signals to the flashmemory FMRY are designated. In addition, the control circuit CONTsuppresses substantial operation of built-in function blocks having nodirect relation to rewrite control by means of the general purpose PROMwriter PRW, such as the central processing unit CPU, random accessmemory RAM and mask read only memory MASKROM. For example, connection ofsuch a built-in function block as the central processing unit CPU to theflash memory FMRY is disconnected through switch means SWITCHrespectively arranged in data bus DBUS and address bus ABUS, asexemplified in FIG. 3. The switch means SWITCH can also be grasped astristate type output circuits arranged in a circuit for delivering datafrom the built-in function block such as the CPU to the data bus DBUSand in a circuit for delivering addresses to the address bus ABUS. Suchtristate output circuits are controlled such that they are brought intohigh impedance condition in response to the second operation mode. Inthe example of FIG. 3, the built-in function blocks not being directlyrelated to the rewrite control based on the general purpose PROM writer,such as the central processing unit CPU, random access memory RAM andmask read only memory MASKROM, are brought into a low power consumptionmode by a standby signal STBY*^(★) (sign*^(★) means that a signalassigned with this sign is a low active signal). If in a low powerconsumption mode the tristate output circuits are so controlled as to bebrought into high output impedance condition, then the power consumptionmode may be set to those function blocks in response to designation ofthe second operation mode by means of the mode signals MD0 to MD2 tosubstantially suppress operation of the built-in function blocks nothaving direct relation to the rewrite control based on the generalpurpose PROM writer PRW, such as the CPU, RAM and ROM.

[0139] When the second operation mode is set, the I/O ports PORTdata,PORTaddr and PORTcont of the microcomputer MCU couple to the generalpurpose PROM writer PRW through a conversion socket SOCKET. Theconversion socket SOCKET has on the one hand a terminal arrangementadapted for the I/O ports PORTdata, PORTaddr and PORTcont and on theother hand a terminal arrangement adapted for a standard memory,terminals of one terminal arrangement having the same functions as thoseof terminals of the other terminal arrangement being connected mutuallyinside the conversion socket SOCKET.

[0140] [4] A Write Control Program Under the Control of a CPU

[0141]FIG. 4 is a block diagram drawn from the viewpoint of rewrite of aflash memory based on CPU control. In the microcomputer MCU of FIG. 1, arewrite control program to be executed by the central processing unitCPU has precedently been written in the flash memory FMRY by means ofthe general purpose PROM writer PRW. In the microcomputer MCU of FIG. 2,a rewrite control program to be executed by the central processing unitCPU can be held in the mask read only memory MASKROM. When the firstoperation mode is designated by the mode signals MD0 to MD2 and adecoder included in the control circuit CONT recognizes thisdesignation, the central processing unit CPU carries out write of datainto the flash memory FMRY in accordance with a write control programwritten in the flash memory FMRY in advance or the rewrite controlprogram held in the mask read only memory MASKROM.

[0142]FIG. 5 shows a memory map of the microcomputer having the alloverflash memory (see FIG. 1). In the figure, a rewrite control program anda transfer control program have precedently been written inpredetermined areas of the flash memory. When the first operation modeis designated, the central processing unit CPU executes the transfercontrol program to transfer the rewrite control program to the randamaccess memory RAM. After completion of the transfer, the processing ofthe central processing unit CPU branches to execution of the rewritecontrol program on the random access memory RAM and through this, theerase and write (inclusive of verification) of the flash memory FMRY isrepeated.

[0143]FIG. 6 shows a memory map of the microcomputer having the mask ROMalong with the flash memory (see FIG. 2). In this case, the transferprogram as explained with reference to FIG. 5 is unneeded. When thefirst operation mode is designated, the central processing unit CPUsequentially executes a rewrite control program held in the mask readonly memory MASKROM to thereby repeat erase and write of the flashmemory FMRY.

[0144]FIG. 7 shows an example of the control procedure of erase by thecentral processing unit CPU. Firstly, in accordance with the rewritecontrol program, the central processing unit CPU performs pre-write ofmemory cells present within an address range to be erased (steps 71 to74). Through this, states of the memory cells before erase are alluniformed to written states. Subsequently, memory cells standing forobjects to be erased are erased little by little while verifying thedegree of erase each time erase is completed (erase/verify) in order toprevent excessive erase, thus completing an erase operation (steps 75 to79). Erase by means of the general purpose PROM writer PRW can be donein a similar way. Erase sequence for the flash memory will be detailedhereinafter with reference to FIG. 22.

[0145]FIG. 8 shows an example of the control procedure of write by thecentral processing unit CPU. Firstly, the central processing unit CPUsets a write start address of the flash memory FMRY (step 81).Subsequently, data transmitted from the outside is read throughperipheral circuits designated by the rewrite control program, forexample,the serial communication interface SCI or I/O ports (step 82).The thus read data is written into the flash memory FMRY for apredetermined time (step 83) and the written data is read to verifywhether the data is written normally (write/verify) (step 84).Thereafter, the above read, write and verify of data are repeated untilthey are completed for a write end address (steps 85 and 86). Write bymeans of the general purpose PROM writer can be done in a similar way.In this case, however, data to be written is supplied from the PROMwriter PRW through predetermined ports. Write sequence for the flashmemory will be detailed later with reference to FIG. 21.

[0146] [5] Proper use of Write by the General Purpose PROM Writer orWrite by the CPU Control

[0147] Principally, write by the general purpose PROM writer is appliedto write of initial data or an initial program used before on-board ofthe microcomputer MCU, that is, mounting of the microcomputer MCU into asystem. This can ensure that a relatively large amount of informationcan be written efficiently.

[0148] Write based on the CPU control is applied to the case wheretuning of data is carried out while operating the system on which themicrocomputer MCU is mounted (called a mounting machine) or the casewhere changes of data and programs under condition that themicrocomputer MCU is mounted on the system (on-board condition), such asbug countermeasures for programs or changes of programs concomitant withversion-up of the system, are needed. Through this, the flash memoryFMRY can be rewritten without removing the microcomputer MCU from themounting system.

[0149] [6] Expedient to Achieve Rewrite on Real Time Base

[0150]FIG. 9 shows an example of a technique of expedient to rewrite theflash memory on real time base. In the flash memory, because of itsstorage formatting, time required for erase cannot be reduced even whenthe storage capacity of a memory block defined as a simultaneous erasingunit is made to be small and it amounts up to, for example, several oftens of milliseconds to several of seconds. This makes it difficult toperform tuning of data by rewriting control data held in the flashmemory on real time base while operating the system with themicrocomputer MCU mounted thereon. To cope with this problem, thebuilt-in RAM is utilized as a work area or data buffer area for rewriteof memory block.

[0151] More particularly, data of a predetermined small memory block SMBholding data to be subjected to tuning is first transferred to aspecified address area of the random access memory RAM.

[0152] Next, the microcomputer MCU is switched to a flash memory writemode of operation. This write operation mode is set either by settingMD0 to MD2 for a predetermined value or by setting the rewrite highvoltage Vpp to be applied to an external terminal to a predeterminedrewrite high voltage. This write mode indicates that the flash memoryFMRY is in a state in which it can be written by the CPU and does notindicate that the CPU is writing the flash memory.

[0153] Subsequently, a specified address area of the random accessmemory RAM is overlapped with an address of a part of the predeterminedsmall memory block SMB of the flash memory FMRY (step 91).

[0154] The reasons why a specified address area of the random accessmemory RAM is overlapped with an address area of a predetermined smallmemory block SMB of the flash memory FMRY are as follows. Namely, whenthe flash memory FMRY stores a user program (e.g., an engine controlprogram) and user data (e.g., engine control data), the user data areread out by the CPU in the course of execution of the user program tothereby effect engine control on the basis of the read out data. Thus,the user program contains addresses of a storage area of the flashmemory FMRY in which the user data are stored. Therefore, in order tochange the user data (i.e., in order to effect the user data tuning)without converting the user program, that is, without rewriting theaddresses of the user data contained in the user program, it isnecessary, for example, to change the address location of a specifiedaddress area of the random access memory RAM in such a manner that thespecified address area of the random access memory RAM an be considered,when viewed in the address space of the CPU, as being the same as apredetermined address area of the flash memory FMRY in which the userdata to be subjected to tuning are stored.

[0155] In other words, for tuning of user data, the addresses of thespecified address area of the random access memory RAM are changed tothe addresses of an area of the flash memory FMRY in which the user datato be subjected to tuning are stored. When the CPU executing the userprogram accesses the user data (data being subjected to tuning) in theflash memory FMRY, actually, the flash memory is not accessed but thespecified area of the random access memory RAM is accessed.

[0156]FIG. 63 is a diagrammatic representation of overlap of thespecified address area of the random access memory RAM with thepredetermined address area of the flash memory FMRY.

[0157] For example, as shown in FIG. 63, the address space of the flashmemory FMRY is represented to be between hexadecimal addresses E000 andEE7F, and the address space of the random access memory RAM (built-inRAM area) is represented to be between hexadecimal addresses F680 andFE7F. In this figure, “H′” indicates that the addresses are in ahexadecimal representation.

[0158] The specified address area (SRA) of the random access memory RAMis, for example, a 128 byte-area having addresses from H′F680 to H′F6FF.When user data tuning is carried out, the addresses of the specifiedaddress area (SRA) are, in FIG. 63, overlapped with addresses D′ECOO toH′EC7E of a part of the address area (H′ECOO to H′ECFF) of the smallmemory block SMB within the address space of the flash memory FMRY.

[0159] Such a change of address arrangement can be realized by makingthe decode logic of the random access memory RAM switchable in responseto a predetermined control bit or setting of a flag.

[0160] Namely, the chip select controller CSCONT shown in FIG. 14 has aRAM control register RAMCR as shown in FIG. 64. This register is aneight bit register which is readable and writable from the CPU and inwhich each of bits 3 to 0 has an initial value “0” and each of the bits7 to 4 has an initial value “1”. The bits 3 to 0 are made valid when themicrocomputer MCU is in a write operation mode. Here, the initial valuesare those set in the register at the time when the microcomputer isreset.

[0161] The bit 3 (RAMS) of the RAM control register RAMCR determines ithow the specified address area SRA of the random access memory RAM isutilized, i.e., whether the specified address area SRA of the randomaccess memory is utilized as having its original addresses or as havingaddresses overlapped with those of a part of the address area of thesmall address block SMB of the flash memory FMRY.

[0162]FIG. 65 illustrates how to set addresses of the specified addressarea SRA by use of the RAM control register RAMCR. When the RAMS bit isreset to “0”, the specified address area SRA is utilized as having itsoriginal addresses H′F680 to H′F6FF of the random access memory RAM,while when the RAMS bit is set to “1” with the microcomputer CPU beingunder write operation mode, the specified address area SRA is utilizedas being overlapped with the addresses of a part of the small memoryblock SMB of the flash memory FMRY.

[0163] Bit 2 (RAM2) to bit 0 (RAM0) of the RAM control register RAMCRserve to determine it where in the small memory block SMB of the flashmemory FMRY the addresses of the specified area SRA should be overlappedwith. When the RAMS bit is reset (cleared) to “0”, the values of thebits 2 to 0 are of no significance.

[0164] Meanwhile, when the RAMS bit is set to “1”, the addresses of thespecified area SRA are variable depending on the values of the bits 2 to0.

[0165] Namely, the addresses of the specified area SRA are:

[0166] H′ECOO to H′EC7F for bits 2 to 0 being “0”, “0”, “0”,

[0167] H′EC80 to H′ECFF for bits 2 to 0 being “0”, “0”, “1”,

[0168] H′ED00 to H′ED7F for bits 2 to 0 being “0”, “1”, “0”,

[0169] H′ED80 to H′EDFF for bits 2 to 0 being “0”, “1”, “1” and

[0170] H′EE00 to H′EE7F for bits 2 to 0 being “1”, “0”, “0”.

[0171] Then, tuning of control data is carried out using the specifiedaddress area SRA of the random access memory RAM which is overlappedwith the address of the predetermined memory block.

[0172] After completion of tuning (step 92), the address overlapping ofthe random access memory RAM and memory block SMB is released, so thatthe address arrangement of the random access memory RAM restores itsoriginal state (step 93). Namely, the value of the RAMS bit of the RAMcontrol register RAMCR is changed by the CPU from “1” (set state) to “0”(reset or cleared state).

[0173] Finally, data having been subjected to tuning which is held inthe the specified address area SRA of the random access memory RAM iswritten by the CPU into the predetermined address area of the memoryblock SMB of the flash memory FMRY (step 94).

[0174] This writing operation is performed by executing the writecontrol program stored in the flash memory FMRY. Namely, data in thesmall memory block of the flash memory FMRY which data are to be changedor tuned are erased in accordance with the steps of the erase flowchart, as will be later described in detail with reference to FIG. 22.Thereafter, the data having been transferred to the random access memoryRAM and having been subjected to the tuning are written by the CPU intothe small memory block SMB of the flash memory FMRY (the data beingwritten in the whole of the the small memory block SMB). In other words,the scheme which will be next described in “[7] Making partial rewriteof a memory block efficient” is employed.

[0175] Through this, the same data as control data held in the flashmemory which has been rewritten on real time base can eventually be setin or obtained on the memory block SMB while operating the system withthe microcomputer MCU mounted thereon.

[0176] Subsequently, the microcomputer MCU is switched from the flashmemory write operation mode to the normal operation mode by resettingthe mode signal MD0 to MD2 or by resetting the rewrite high voltage Vppto be applied to the external terminal of the microcomputer to 0 volt.FIG. 66 shows an example of a circuit structure of a part of the chipselect controller CSCONT in which a RAM address decoder RADE and a flashaddress decoder FADE are coupled to address bus lines L15 to L7 for thehigher bits 15 to 7 of the inner address bus (bit 15 to 0) to decode theaddress signal on the address bus lines L15 to L7. For example, if theaddress signal on the lines L15 to L7 represents an address area of therandom access memory RAM, the RAM address decoder RADE decodes thesignal and makes its output signal RADES high. Meanwhile, if the addresssignal on the lines L15 to L7 represents an address area of the flashmemory FMRY, the flash address decoder FADE decodes the signal and makesits output signal FADES high.

[0177] In FIG. 66, the logic circuit OCC other than the RAM addressdecoder RADE and the flash address decoder FADE serves as means foroverlapping the addresses of the predetermined address area of the flashmemory FMRY (a part of the small memory block SMB) with addresses of thespecified address area of the random access memory RAM, as describedabove. The RAMS and RAM2 to RAM0 correspond to those of the RAM controlregister RAMCR described above with reference to FIG. 64.

[0178] A specific operation of the circuit shown in FIG. 66 isconsidered to be readily understood by those skilled in the art and willnot be described in detail. With the RAMS bit value being made “1”, whenthe addresses of the predetermined address area of the flash memory FMRY(the addresses overlapped with the addresses of the specified addressarea SRA of the random access memory RAM) are accessed under theconditions in which the values of RAM2 to RAM0 are made to correspond toone of states (2) to (6) shown in FIG. 65, a detection at a low levelindicating that the overlap designation area is accessed appears on apoint NOD in FIG. 66. As a result, flash memory select signal MS-FLN^(★)is brought into a high level non-active state and RAM selection signalMS-RAMN^(★) is brought into a low level active state so that the addresssignal for otherwise accessing the small memory block SMB of the flashmemory FMRY now serves to access the specified address area SRA of therandom access memory RAM without accessing the small memory block SMB ofthe flash memory FMRY.

[0179] On the other hand, with the RAMS bit value being set to “1”, whenthe address signal on the address lines L15 to L7 indicates an addressother than one of the addresses of address areas of the flash memoryFMRY represented by bits RAM2 to RAM0 of the RAM control register RAMCR(i.e., other than the address area of one of the addresses representedby one of states (2) to (6) shown in FIG. 65), the detection signal onthe node NOD is brought into a high level. As a result, when the addresssignal on the address bus lines indicates the flash memory FMRY, theflash memory selection signal MS-FLN^(★) is brought into a low levelactive state so that data is read into the CPU from an address area ofthe flash memory FMRY specified by the address signal on the address buslines, while when the address signal on the address bus lines indicatesthe random access memory RAM, the RAM selection signal MS-RAMN^(★) isbrought into a low level active state so that an address area of therandom access memory RAM specified by the address signal on the addressbus lines is accessed by the CPU for data reading therefrom or datawriting thereinto.

[0180] [7] Making Partial Rewrite of a Memory Block Efficient

[0181]FIG. 10 shows an example of a technique of making partial rewriteof a memory block of the flash memory efficient. When part ofinformation held in a predetermined memory block of the flash memory isrewritten upon modification of a bug of a program or version-up of theprogram, information held in the memory block having a smaller storagecapacity than that of the RAM is transferred to the built-in RAM (step101), part of the transferred information is renewed on the RAM (step102), and after the data in the memory block in question is erased (step103), the memory block in is rewritten with the renewed information(step 104). Through this, even when one of the memory blocks SMB iserased simultaneously, information held in that memory block SMB can bepreserved. Therefore, by receiving only data to be rewritten from theoutside and rewriting the data on the RAM, transfer, from the outside,of information not required to be rewritten and held in the flash memoryFMRY before rewriting can be unneeded, thus avoiding wastefulness ofinformation transfer for partial rewrite of the memory block. The abovedescription with reference to FIG. 10 will be clear also from thedescription with reference to FIGS. 63 to 66 made hereinbefore.

[0182] [8] The Principle of the Flash Memory

[0183]FIGS. 11A and 11B show the principle of the flash memory. A memorycell exemplified in FIG. 11A is constructed of an insulated gate fieldeffect transistor having a two-layer gate structure. In the FIG. 1designates a P type silicon substrate, 14 a P type semiconductor regionformed in the silicon substrate 1, 13 an N type semiconductor region and15 and N type semiconductor region of low concentration. Denoted by 8 isa floating gate formed over the P type silicon substrate 1 through athin oxide film 7 (for example, having a thickness of 10 nm) serving asa tunnel insulating film and by 11 is a control gate formed over thefloating gate 8 through an oxide film 9. A source is formed of 13 and 15and a drain is formed of 13 and 14. Information stored in this memorycell is substantially held as a change in threshold voltage in thetransistor. Described hereinafter is the case where a transistor used inthe memory cell to store information (hereinafter referred to as amemory transistor) is of N channel type, except otherwise described.

[0184] A write operation of information to the memory cell can berealized by, applying high voltages to, for example, the control gate 11and drain so that electrons may be injected from the drain side to thefloating gate 8 through avalanche injection. As a result of the writeoperation, the memory transistor assumes a threshold voltage as viewedfrom its control gate 7 which is raised as shown in FIG. 11B incomparison with that of the memory transistor without subjected to awrite operation and so placed in the erase condition.

[0185] On the other hand, an erase operation can be realized by applyinga high voltage to, for example, the source so that electrons may bedrawn out of the floating gate 8 to the source side through tunnelphenomenon. As a result of the erase operation, the memory transistorassumes a threshold voltage as viewed from its control gate 11 which islowered as shown in FIG. 11B. The threshold of the memory transistorshown in FIG. 11B is set to a positive voltage level in both of thewrite condition and erase condition. In other words, in relation to aword line selection level applied from a word line to the control gate11, the threshold voltage for write condition is set to be higher andthe threshold voltage for erase condition is set to be lower. Thanks tothe fact that both the threshold voltages are related to the word lineselection level in a manner described as above, a memory cell can beconstructed of a single transistor without employing a selectiontransistor. When stored information is to be erased electrically, eraseof the stored information can be done by drawing electrons stored in thefloating gate 8 to the source electrode and therefore, by keeping theerase operation continuing for a relatively long time, electrons whichare larger in amount than electrons injected in the floating gate 8 upona write operation are drawn out. Thus, when electrical erase keepscontinuing for a relatively long time, resulting in excessive erase, thethreshold voltage of the memory transistor assumes, for example, anegative level and there occurs such inconvenience that a word line isselected even when the word line is at an unselection level. Write canalso be effected by utilizing tunnel current as in the case of erase.

[0186] In a read operation, to prevent the memory cell from beingwritten weakly, that is, to prevent the floating gate 8 from beinginjected with undesired carriers, voltages applied to the drain andcontrol gate 11 are limited to relatively low values. For example, a lowvoltage of about 1V is applied to the drain and a low voltage of about5V is applied to the control gate 11. Under the application of thesevoltages, the magnitude of a channel current flowing through the memorytransistor is detected so as to decide whether information stored in thememory cell is “0” or “1”.

[0187]FIG. 12 shows the principle of construction of a memory cell arrayusing the memory transistors. In the figure, four memory transistors(memory cells) Q1 to Q4 are typically depicted. In the memory cellsarranged in matrix in X and Y directions, control gates (gates forselection of memory cells) of memory transistors Q1 and Q2 (Q3 and Q4)arranged on the same row are connected to a corresponding word line WL1(WL2), and drain regions (input/output nodes of memory cells) of memorytransistors Q1 and Q3 (Q2 and Q4) are connected to a corresponding dataline DL1 (DL2). Source regions of the memory transistors Q1 and Q3 (Q2and Q4) are coupled to a source line SL1 (SL2).

[0188] Table 1 shows an example of voltage conditions for eraseoperation and write operations of memory cells. TABLE 1 selection/memory element unselection source drain gate Writing Q1 selection 0 V 6V 12 V Q2 unselection 0 V 0 V 12 V Q3 unselection 0 V 6 V 0 V Q4unselection 0 V 0 V 0 V Erasing (positive voltage scheme) Q1, Q3selection 12 V 0 V 0 V Q2, Q4 unselection 0 V 0 V 0 V Erasing (negativevoltage scheme) Q1, Q2 selection 5 V 0 V −10 V Q3, Q4 unselection 5 V 0V 0 V

[0189] In this table, memory elements mean memory cells and gates meancontrol gates serving as selection gates of the memory cells. In erasebased on a negative voltage scheme shown in the figure, a negativevoltage of, for example, −10 V is applied to the control gate to form ahigh electric field necessary for erase. As is clear from the voltageconditions exemplified in the figure, in erase based on a positivevoltage scheme, memory cells at least sources of which are connected incommon can be erased collectively. Accordingly, with the source linesSL1 and SL2 connected together in the construction of FIG. 12, the fourmemory cells Q1 to Q4 can be erased simultaneously. In this case, bychanging the number of memory bits connected to the same source line,the size of memory block can be set desirably. As for the source linedivision scheme, in addition to a typical instance shown in FIG. 12where the data line is defined as a unit (a common source line is solaid as to extend in the data line direction), there is available aninstance where the word line is defined as a unit (a common source lineis so laid as to extend in the word line direction). On the other hand,in erase based on the negative voltage scheme, memory cells havingcontrol gates connected in common can be erased simultaneously.

[0190] [9] Formation of a Plurality of Memory Blocks Having DifferentStorage Capacities and Defined Each in a Unit of Data Line

[0191]FIG. 13 is a circuit block diagram showing an example of a flashmemory in which storage capacities of simultaneously erasable memoryblocks are made to be different.

[0192] The flash memory FMRY shown in the figure has data input/outputterminals D0 to D7 of 8 bits, so that memory array portions ARY0 to ARY7are provided in association with the respective data input/outputterminals. Each of the memory array portions ARY0 to ARY7 is dividedinto two of a memory block LMB having a relatively large storagecapacity and a memory block SMB having a relatively small storagecapacity. In the figure, details of the memory array portion APY0 aretypically illustrated and the other memory array portions ARY1 to ARY7are constructed similarly.

[0193] In each of the memory array portions ARY0 to ARY7, memory cellsMC formed of insulated gate field effect transistors of two-layer gatestructure as explained previously in connection with FIG. 11 arearranged in matrix. Also, in the figure, WL0 to WLn designate word lineswhich are common to all of the memory array portions ARY0 to ARY7.Control gates of memory cells arranged on the same row are connected toa corresponding word line. In each of the memory array portions ARY0 toARY7, drain regions of memory cells MC arranged on the same column areconnected to a corresponding data line DL0, . . . or DL7. Source regionsof memory cells MC constituting the memory block SMB are connected incommon to a source line SL1 and source regions of memory cells MCconstituting the memory block LMB are connected in common to a sourceline SL2.

[0194] A high voltage Vpp used for erase is supplied from voltage outputcircuits VOUT1 and VOUT2 to the source lines SL1 and SL2. An outputoperation of the voltage output circuits VOUT1 and VOUT2 is selected inaccordance with values of bits B1 and B2 of an erase block designationregister. For example, by setting “1” to the bit B1 of the erase blockdesignation register, only the memory block SMB of each of the memoryarray portions ARY0 to ARY7 is allowed to be erased simultaneously. When“1” is set to the bit B2 of the erase block designation register, onlythe memory block LMB of each of the memory array portions ARY0 to ARY7is allowed to be erased simultaneously. When “1” is set to both the bitsB1 and B2, the whole of the flash memory is allowed to be erasedsimultaneously.

[0195] Selection of the word line WL0, . . . or WLn is effected bycausing a row address decoder XADEC to decode a row address signal AXfetched in through a row address buffer XABUFF and a row address latchXALAT. A word driver WDRV selects a word line on the basis of aselection signal delivered out of the row address decoder XADEC. In adata read operation, the word driver WDRV is operated with a powersupply as represented by voltage Vcc such as 5 V and earth potentialsuch as 0 V fed from a voltage selection circuit VSEL, and it drives aword line to be selected to a selection level by the voltage Vcc andmaintains a word line not to be selected at an unselection level such asearth potential. In a data write operation, the word driver WDRV isoperated with a power supply as represented by voltage Vpp such as 12 Vand earth potential such as 0 V fed from the voltage selection circuitVSEL and it drives a word line to be selected to a write high voltagelevel such as 12 V. In a data erase operation, the output of the worddriver WDRV is rendered to be a low voltage level such as 0 V.

[0196] In each of the memory array portions ARY0 to ARY7, the data linesDL0 to DL7 are connected in common to a common data line CD throughcolumn selection switches YS0 to YS7. Switching control of the columnselection switches YS0 to YS7 is carried out by causing a column addressdecoder YADEC to decode a column address signal AY fetched in through acolumn address buffer YABUFF and a column address latch YALAT. An outputselection signal of the column address decoder YADEC is fed in common toall of the memory array portions ARY0 to ARY7. Accordingly, when any oneof the output selection signals of the column address decoder YADEC isrendered to be a selection level, a single data line is brought intoconnection to the common data line CD in each of the memory arrayportions ARY0 to ARY7.

[0197] Data read from a memory cell MC to the common data line isapplied to a sense amplifier SAMP through a selection switch RS and theamplified data is delivered to the outside through a data output latchDOLAT and a data output buffer DOBUFF. The selection switch RS is set toa selection level in synchronism with a read operation. Write data fedfrom the outside is held in a data input latch circuit DILAT through adata input buffer DIBUFF. When data stored in the data input latchcircuit DILAT is “0”, a write circuit WRIT supplies a write high voltageto the common data line CD through a selection switch WS. This writehigh voltage is fed to the drain of a memory cell, whose control gate isapplied with the high voltage under the direction of a row addresssignal AX, through a data line selected by a column address signal AY,so that write into the memory cell in question is carried out. Theselection switch WS is set to a selection level in synchronism with awrite operation. Various timings for write and erase and voltageselection control signals are generated by a write/erase control circuitWECONT.

[0198] [10] Details of a Microcomputer Corresponding to FIG. 1

[0199]FIG. 14 is a block diagram showing an embodiment of furtherdetails of a microcomputer corresponding to the microcomputer of FIG. 1.The microcomputer MCU shown in the figure comprises as the same functionblocks as those shown in FIG. 1 a central processing unit CPU, a flashmemory FMRY, a serial communication interface SCI, a control circuitCONT and a random access memory RAM. As equivalence to the timer of FIG.1, the microcomputer comprises a 16-bit integrated timer pulse unit IPUand a watchdog timer WDTMR. Also, as equivalence to the peripheralcircuits I/O of FIG. 1, the microcomputer comprises ports PORT1 toPORT12. Further, as the other function blocks, there are provided aclock oscillator CPG, an interruption controller IRCONT, ananalog/digital converter ADC and a wait state controller WSCONT. Thecentral processing unit CPU, flash memory FMRY, random access memory RAMand 16-bit integrated timer pulse unit IPU are coupled to an address busABUS,a lower data bus LDBUS (for example, 8 bits) and an upper data busHDBUS (for example, 8 bits). The serial communication interface SCI,watchdog timer WDTMR, interruption controller IRCONT, analog/digitalconverter ADC, wait state controller WSCONT, chip select controllerCSCONT and ports PORT1 to PORT12 are coupled to the address bus ABUS andhigher data bus HDBUS.

[0200] In FIG. 14, the chip select controller CSCONT, an example of acircuit structure is shown in FIG. 66, serves as means for decodinghigher bits (for example, bits 15 to 7) of the address bus (ABUS) togenerate a selection signal indicating which address area the addresssignal on the address bus (ABUS) designates for selection. The chipselect controller CSCONT may include a plurality of address areadesignation registers which are set for designating address areas of thebuilt-in random access memory RAM, address areas of the built-in flashmemory, address areas of I/O devices such as a memory and/or a floppydisk controller externally connected to the microcomputer MCU.

[0201] When the access speed of the I/O devices such as a memory and/ora floppy disk controller externally connected to the microcomputer islower than the access speed of the microcomputer, the chip selectcontroller CSCONT decodes the address signal on the address bus todetermine whether the address signal is for an access to a low speedmemory or an I/O device externally connected to the microcomputer MCU.If so determined, the the chip select controller CSCONT informs the waitstate controller WSCONT thereof. As a result, the wait state controllerWSCONT inserts one or more wait states in the bus cycle of themicrocomputer. Such chip select controller as described above isdisclosed, for example, U.S. Pat. No. 5,070,473 issued on Dec. 3, 1991and assigned to Hitachi Microcomputer Engineering Ltd. and Hitachi,Ltd., the disclosure of which is hereby incorporated by reference.

[0202] In FIG. 14, Vpp designates a high voltage for rewrite of theflash memory. EXTAL and XTAL represent signals supplied from a not-shownvibrator, provided externally of the chip of the microcomputer, to theclock oscillator CPG. Denoted by φ is a synchronizing clock signaldelivered from the clock oscillator CPG to the outside. MD0 to MD2designate mode signals supplied to the control circuit CONT in order toset the first operation mode or the second operation mode upon rewriteof the flash memory FMRY and correspond to the mode signal MODE inFIG. 1. Denoted by RES^(★) is a reset signal and by STBY^(★) is astandby signal, these signals being supplied to the central processingunit CPU and other circuit blocks. NMI designates a non-maskableinterrupt signal for applying a non-maskable interruption to theinterruption controller ICONT. Other interruption signals, not shown,are applied to the interruption controller ICONT through the ports PORT8and PORT9. Denoted by AS^(★) is an address strobe signal indicative ofvalidity of an address signal delivered to the outside, by RD^(★) is aread signal for informing the outside of a read cycle, by HWR^(★) is anupper byte write signal for informing the outside of a write cycle ofupper 8 bits and by LWR^(★) is a lower byte write signal for informingthe outside of a write cycle of lower 8 bits, these signals being accesscontrol signals for the outside of the microcomputer MCU.

[0203] The ports PORT1 and PORT2 are allotted, though not limitedly, forinput/output of data BD0 to BD15 used for the microcomputer MCU toaccess the outside in the other mode than the second operation mode inwhich the external PROM writer performs direct rewrite control of theflash memory FMRY. At that time, the ports PORT3 to PORT 5 are allotted,though not limitedly, for address signals BA0 to BA19.

[0204] On the other hand, when the second operation mode is set in themicrocomputer MCU, the ports PORT2 to PORT5 and PORT8 are allotted,though not limitedly, for connection to the PROM writer adapted tocontrol write of the flash memory FMRY. More specifically, the portPORT2 is allotted for input/output of data ED0 to ED7 for write andverify and the ports PORT3 to PORTS and PORT8 are allotted for input ofaddress signals EA0 to EA16 as well as input of access control signalCE^(★) (chip enable signal), OE^(★) (output enable signal) and WE^(★)(write enable signal). The chip enable signal CE^(★) is an operationselection signal for the flash memory FMRY delivered from the PROMwriter, the output enable signal OE^(★) is a designation signal of anoutput operation for the flash memory FMRY and the write enable signalWE^(★) is a designation signal of a write operation for the flash memoryFMRY. For inputting one-bit EA9 of address signals EA0 to EA16, theinput terminal of the signal NMI is allotted. External terminals of thethus allotted ports and other necessary external terminals including theapplication terminal of high voltage Vpp are connected to the generalpurpose PROM writer through the conversion socket SOCKET explained inconnection with FIG. 3. Conveniently, the allotment of the externalterminals at that time can be done in the form of such a terminalarrangement that the microcomputer MCU can be connected with ease to thePROM writer PRW through the conversion socket SOCKET. The externalterminals allotted for connection to the PROM writer PRW in the secondoperation mode are assigned with other functions in the other operationmode of the microcomputer MCU.

[0205]FIG. 15 shows a top view of a flat package of the FIG. 14microcomputer MCU which is, for example, sealed with resin and designedto have external terminals in four directions. Signals shown in FIG. 15are identical to those shown in FIG. 14. External terminals (pins) notassigned with signal names may be utilized as an input pin of a waitsignal, an input pin of a bus request signal, an output pin of a busacknowledge signal and input/output pins for signals between such aperipheral circuit as serial communication interface SCI and theoutside.

[0206] In the package FP shown in FIG. 15, the distance PP betweenadjacent terminals (pins) led from the package FP may be 0.5 mm or less.To explain, consider that a user of the microcomputer MCU connects theflash memory FMRY included in the microcomputer MCU to the PROM writerPRW through the conversion socket SOCKET so as to write data into theflash memory FMRY. In this case, when the distance between adjacentterminals (pin pitch) PP of the package FP is set to be 0.5 mm or less,pin bends due to unwanted contact between external terminals of theconversion socket SOCKET and those of the package FP tend to occur whenthe package FP is inserted into the conversion socket SOCKET. In theevent that such pin bends take place, electrical connection betweenterminals of the conversion socket SOCKET and those of the package FP isprevented in respect of terminals suffering from the pin bends, thusmaking it impossible to write data into the flash memory FMRY by meansof the PROM writer PRW.

[0207] As far as this point is concerned, according to the invention,the central processing unit CPU is allowed to write data into the flashmemory FMRY and therefore, after the package of the microcomputer MCU ismounted on a mounting board (printed board), the user can write data inthe flash memory FMRY by means of the central processing unit CPUwithout using the external PROM writer PRW for write of data into theflash memory FMRY, whereby even when the microcomputer MCU is sealed inthe package having the pin pitch which is 0.5 mm or less, the user canbe relieved from committing lead bends of external terminals led fromthe package. It is to be noted that in the semiconductor maker, anautomatic handler can be used and therefore a test of the microcomputerMCU can be conducted steadily and without causing pin bends even if themicrocomputer MCU is sealed in the package of the pin pitch being 0.5 mmor less.

[0208] [11] A Control Circuit for Rewrite of Flash Memory FMRY

[0209]FIG. 16 is a block diagram showing the whole of the flash memoryFRMY incorporated in the microcomputer MCU of FIG. 14. In the figure,ARY designates a memory array in which memory cells formed of insulatedgate field effect transistors of two-layer gate structure explained inconnection with FIGS. 11A and 11B are arranged in matrix. In this memoryarray ARY, like the construction explained with reference to FIG. 13,control gates of memory cells are connected to a corresponding wordline, drain regions of memory cells are connected to a correspondingdata line and source regions of memory cells are connected to a sourceline common to each memory block, but the array is divided into memoryblocks in a different manner from FIG. 13. For example, as shown in FIG.17, the array is divided into seven large memory blocks (large blocks)LMB0 to LMB6 each having a relatively large storage capacity and eightsmall memory blocks (small blocks) SMB0 to SMB7 each having a relativelysmall storage capacity. The large memory block is utilized as, forexample, a program storing area or a large capacity data storing area.The small memory block is utilized as, for example, a small capacitydata storing area.

[0210] In FIG. 16, ALAT designates a latch circuit for address signalsPAB0 to PAB15. In the first operation mode, the address signals PAB0 toPAB15 correspond to output address signals BA0 to BA15 of the centralprocessing unit CPU. In the second operation mode, the address signalsPAB0 to PAB15 correspond to output address signals EA0 to EA15 of thePROM writer PRW. XADEC designates a row address decoder for decoding arow address signal fetched in through the address latch ALAT. WDRVdesignates a word driver for driving a word line on the basis of aselection signal delivered out of the row address decoder XADEC. In adata read operation, the word driver WDRV drives the word line with avoltage of 5 V and in a data write operation, drives the word line witha high voltage of 12 V. In a data erase operation, all outputs of theword driver WDRV are rendered to be a low voltage level of OV. YADECdesignates a column address decoder for decoding a column address signalfetched in through the address latch YALAT. YSEL designates a columnselection circuit for selecting a data line in accordance with an outputselection signal of the column address decoder YADEC. SAMP designates asense amplifier for amplifying a read signal from a data line selectedby the column selection circuit YSEL in a data read operation. DOLATdesignates a data output latch for holding an output of the senseamplifier. DOBUFF designates a data output buffer for delivering dataheld in the data output latch DOLAT to the outside. In the figure, PDB0to PDB7 are data of lower 8 bits (one byte) and PDB8 to PDB15 are dataof upper 8 bits. In accordance with this example, the output data is oftwo bytes at maximum. DIBUFF designates a data input buffer for fetchingwrite data fed from the outside. The data fetched in by the data inputbuffer DIBUFF is held in a data input latch circuit DILAT. When the dataheld in the data input latch circuit DILAT is “0”, a write circuit WRITsupplies a write high voltage to a data line selected by the columnselection circuit YSEL. This write high voltage is supplied to the drainof a memory cell whose control gate is applied with a high voltage inaccordance with a row address signal, so that the memory cell inquestion undergoes write. ERASEC designates an erase circuit forsupplying an erase high voltage to a source line of a designated memoryblock to erase the memory block simultaneously.

[0211] FCONT designates a control circuit for performing timing controlof a data read operation and selection control of various timings andvoltages for write and erase. This control circuit FCONT comprises acontrol register CREG including the erase block designation registerMBREG and the program/erase control register PEREG.

[0212]FIG. 18 shows an example of the control register CREG. The controlregister CREG includes a program/erase control register PEREG of 8 bitsand erase block designation registers MBREG1 and MBREGG2 each being of 8bits. In the program/erase control register PEREG, Vpp represents a highvoltage applying flag which is rendered to be “1” in accordance with theapplication of a rewrite high voltage. An E bit is a bit for designatingan erase operation and an EV bit is a bit for designating a verifyoperation for erasure. A P bit is a bit for designating a writeoperation (program operation) and a PV bit is a bit for designating averify operation for writing. The erase block designation registerMBREG1 is a register for designating any one of memory blocks containedin the seven divisions of large block and the erase block designationregister MBREG2 is a register for designating any one of memory blockscontained in the eight divisions of small block, each of these registershaving 0-th bit to seventh bit which are bits for designating individualmemory blocks whereby, for example, bit “1” signifies selection of acorresponding memory block and bit “0” signifies unselection of acorresponding memory block. For example, when the seventh bit of theerase block designation register is “1”, the small memory block SMB7 isdesignated.

[0213] The control register CREG is readable/writable from the outside.The control circuit FCONT makes reference to the contents set in thecontrol register CREG to control erase and write in accordance with thecontents. The operation state of erase and write can be controlled bythe CPU or externally by rewriting the contents of the control registerCREG.

[0214] In FIG. 16, the control circuit FCONT is supplied with controlsignals of FLM, MS-FLM, MS-MISN, M2RDN, M2WRN, MRDN, MWRN, IOWOTDN andRST and is also supplied with data of PDB8 to PDB15 of upper one byteand predetermined bits of address signals PAB0 to PAB15.

[0215] The control signal FLM is a signal for designating an operationmode of the flash memory FMRY whereby its “0” designates the firstoperation mode and its “1” designates the second operation mode. Thissignal FLM is formed on the basis of, for example, the mode signals MD0to MD2.

[0216] The control signal MS-FLN is a selection signal of the flashmemory FMRY whereby its “0” designates selection and its “1” designatesunselection. In the first operation mode, the central processing unitCPU delivers the control signal MS-FLN and in the second operation mode,the control signal MS-FLM corresponds to a chip enable signal CE^(★)supplied from the PROM writer PRW.

[0217] The control signal MS-MISN is a selection signal of the controlregister CREG. In this case, selection of which one of the program/erasecontrol register PEREG, erase block designation register MBREG1 anderase block register MBREG2 is determined by consulting predeterminedbits of the address signals PAB0 to PAB15. In the first operation mode,the central processing unit CPU delivers the control signal MS-MISN. Inthe second mode, the most significant address bit EA16 delivered out ofthe PROM writer PRM is deemed as the control signal MS-MISN, though notlimitedly.

[0218] The M2RDN is a memory read strobe signal, the M2WRN is a memorywrite strobe signal, the MRDN is a read signal of the control registerCREG and the MWRN is a write signal of the control register CREG. In thefirst operation mode, the central processing unit CPU delivers thesecontrol signals. In the second operation mode, though not limitedly, awrite enable signal WE^(★) supplied from the PROM writer PRW is deemedas the M2WRN and MWRN and an output enable signal OE^(★) supplied fromthe PROM writer is deemed as the M2RDN and MRDN. The memory write strobesignal M2WRN is deemed as a strobe signal for writing data to be writtenin a memory cell into the data input latch circuit DILAT. Practically,write into the memory cell is started by setting a P bit of the controlregister CREG.

[0219] IOWORDN is a signal for switching access to the flash memory FMRYbetween 8-bit read access and 16-bit read access. In the secondoperation mode, this control signal IOWORDN is fixed to a logical valuefor designating the 8-bit read access.

[0220] RST is a reset signal for the flash memory FMRY. When the flashmemory FMRY is reset by this signal RST or when the Vpp flag ofprogram/erase control register PEREG is rendered to be “0”, the modesetting bits EV, PV, E and P in the program/erase control register PEREGare cleared.

[0221]FIG. 19 is a timing chart showing an example of a memory readoperation in the flash memory FMRY. In the figure, CK1M and CK2M arenon-overlap 2-phase clock signals which are deemed as operationreference clock signals. Denoted by tCYC is cycle time which differsonly slightly from access time for the RAM. A read operation of thecontrol register CREG is carried out at similar timings.

[0222]FIG. 20 is a timing chart showing an example of a memory writeoperation in the flash memory FMRY. In a memory write operationdesignated by a write strobe signal M2WRN shown in the figure, actualwrite of a memory cell is not carried out as described previously butinput address signals PAB0 to PAB15 are held in the address latchcircuit ALAT and input data of PB8 to PB15 is held in the data inputlatch DILAT, thus completing a write cycle of interest. A writeoperation of the control register CREG is conducted at similar timingsbut in this case, actual data write into the control register CREG iscarried out.

[0223] [12] Details of the Rewrite Control Procedure of the Flash MemoryFMRY

[0224] In this item, an example of details of the control procedure willbe described in which the central processing unit CPU or the PROM writerperforms write and erase of the flash memory through the control circuitFCONT. Basically, in the flash memory, information is written into amemory cell under the erase condition. In the first operation mode inwhich rewrite of the flash memory is effected with the microcomputermounted on the system, a rewrite control program to be executed by thecentral processing unit CPU includes a program for erase and a programfor write. The rewrite control program can be programmed such that anerase process routine is initially executed and a write process routineis automatically executed without interruption in accordance withdesignation of the first operation mode. Alternatively, erase and writemay be separated and then the first operation mode may be designatedthereto separately. Rewrite control by the PROM writer can be executedthrough a similar operation to that of the first operation mode. Thewrite control procedure and erase control procedure will now bedescribed.

[0225]FIG. 21 shows an example of details of the write controlprocedure. The procedure shown in the figure is the procedure for writeof data of, for example, one byte and is common to both of the controlby the central processing unit CPU in the first operation mode and thecontrol by the PROM writer in the second operation mode. The followingdescription will be given by way of a control master which isrepresented by the central processing unit CPU.

[0226] In the initial step of data write in a unit of byte, the centralprocessing unit CPU sets one to a counter n built therein (step S1).Subsequently, the central processing unit CPU performs the memory writeoperation explained in connection with FIG. 20 to set data to be writtenin the flash memory FMRY to the data input latch circuit DILAT shown inFIG. 16 and set an address to be written with the data to the addresslatch circuit ALAT (step S2). Then, the central processing unit CPUissues a write cycle to the control register CREG to set a program bit P(step 3). Through this, the control circuit FCONT applies, on the basisof the data and address set in the step 2, high voltages to the controlgate and the drain of a memory cell designated by the address to performwrite. The central processing unit CPU waits for, for example, 10 μsec.to clear the write process time on the flash memory side (step S4) andthen clears the program bit P (step S5).

[0227] Thereafter, in order to confirm the write state, the centralprocessing unit CPU issues a write cycle to the control register CREGand sets a program verify bit PV (step 6). Through this, the controlcircuit CONT utilizes the address set by the step 2 to apply a verifyvoltage to a word line to be selected by that address and to read datafrom the memory cell subjected to write. To ensure a sufficient writelevel, the verify level voltage is set to a voltage level of, forexample, 7 V which is higher than the power supply voltage Vcc such as 5V. Thus, the central processing unit CPU checks coincidence of read-outdata with data utilized for write (step S7). If the central processingunit CPU confirms coincidence through verify, it clears the programverify bit PV (step S8), thus completing write of the one-byte data.

[0228] On the other hand, if the central processing unit CPU confirmsnon-coincidence through verify by step S7, it clears the program verifybit PV in step S9 and thereafter decides whether the value of counter nreaches a write retry upper-limit frequency N (step S10). Thus, if thewrite retry upper-limit frequency N is reached, defective write isdetermined and the processing ends. If the write retry upper-limitfrequency N is not reached, the central processing unit CPU incrementsthe value of counter n by one (step S11) and repeats the processingbeginning with the step S3.

[0229]FIG. 22 shows an example of details of the erase controlprocedure. The procedure shown in the figure is common to both of thecontrol by the central processing unit CPU in the first operation modeand the control by the PROM writer in the second operation mode. Thefollowing description will be given by way of a control master which isrepresented by the central processing unit CPU.

[0230] Upon erase, the central processing unit CPU sets one to itsbuilt-in counter n (step S21). Subsequently, the central processing unitCPU performs pre-write of memory cells within an area to be erased (stepS22). Namely, data “0” is written in a memory cell at an address to beerased. The control procedure for pre-write may make use of the writecontrol procedure explained with reference to FIG. 21. This pre-writeprocessing is carried out in order to uniform, over all bits, electriccharge amounts present in the floating gates before erasing, thus makingthe erase state uniform.

[0231] Subsequently, the central processing unit CPU issues a writecycle to the control register CREG to designate a memory block to beerased simultaneously (step S23). More particularly, a memory blocknumber to be erased is designated to the erase block designationregisters MBREG1 and MBREG2. After the designation of the memory blockto be erased, the central processing unit CPU issues a write cycle tothe control register CREG to set an erase bit E (step 24). Through this,the control circuit FCONT applies a high voltage to a source line of thememory block designated by the step 23 to erase the memory block ofinterest simultaneously. The central processing unit CPU waits for, forexample, 10 msec. to clear the batch erasing process time on the flashmemory side (step S25). A time duration of 10 msec. is shorter than thetime for completing one erase operation. Then, the erase bit E iscleared (step S26).

[0232] Thereafter, in order to confirm the erase state, the centralprocessing unit CPU first sets internally a head address of the memoryblock to be erased simultaneously as an address to be verified (stepS27) and then performs dummy write to the verify address (step S28).Namely, a memory write cycle is issued to the address to be verified.Through this, the memory address to be verified is held in the addresslatch circuit ALAT. Subsequently, the central processing unit CPU issuesa write cycle to the control register CREG and sets an erase verify bitEV (step 29). Through this, the control circuit FCONT utilizes theaddress set by the step 28 to apply an erase verify voltage to the wordline to be selected by the address and to read data of the erased memorycell. To ensure a sufficient erase level, the erase verify voltage isset to a voltage level of, for example, 3.5 V which is lower than thepower supply voltage Vcc such as 5 V. Thus, the central processing unitCPU verifies coincidence of read-out data with data under the erasecompletion state (step S30). If the central processing unit CPU confirmscoincidence through verify, it clears the erase verify bit EV (step S31)and then decides whether the present verify address is a final addressof the erased memory block (step S32), thus completing a series of eraseoperations if the final address is identified. If the final address isnot reached, the central processing unit CPU increments the verifyaddress by one (step S33) and repeats the processing beginning with thestep S29.

[0233] On the other hand, if the central processing unit CPU confirmsnon-coincidence through verify by step S30, it clears the erase verifybit EV in step S34 and thereafter decides whether the value of counter nreaches a gradual erase upper-limit frequency N (step S35). If thegradual erase upper-limit frequency N is reached, a defective erase isdetermined and the processing ends. If the gradual erase upper-limit isnot reached, the central processing unit CPU increments the value ofcounter n by one (step S36) and repeats the processing beginning withstep S24. Practically, in order to prevent an excessive erase in whichthe threshold voltage of the memory cell assumes a negative value owingto erase effected excessively, erase is gradually repeated for a shorttime of 10 msec while performing verify every frequency.

[0234] [13] Formation of a Plurality of Memory Blocks Having DifferentStorage Capacities and Defined Each in a Unit of Word Line

[0235]FIG. 24 shows a memory mat configuration of a flash memory inwhich a plurality of memory blocks are defined each in a unit of wordline and the simultaneously erasable memory blocks have differentstorage capacities.

[0236] While in the configuration shown in FIG. 13 memory blocks aredefined each in a unit of data line, memory blocks are defined in a unitof word line in FIG. 24. In the figure, a memory block LMB having arelatively large storage capacity and a memory block SMB having arelatively small storage capacity are illustrated as representativesthroughout memory array portions ARY0 to ARY7.

[0237] In each of the memory array portions ARY0 to ARY7, memory cellsMC formed of insulated gate field effect transistors of two-layer gatestructure as explained previously in connection with FIG. 11 arearranged in matrix. In the figure, WL0 to WLn designate word lines whichare common to all of the memory array portions ARY0 to ARY7. Controlgates of memory cells arranged on the same row are connected to acorresponding word line. In each of the memory array portions ARY0 toARY7, drain regions of memory cells MC arranged on the same column areconnected to a corresponding data line DL0, . . . or DLm. Source regionsof memory cells MC constituting the small memory block SMB are connectedin common to a source line SLwi extending in the word line direction andsource regions of memory cells MC constituting the large memory blockLMB are connected in common to a source line SLwl extending in the wordline direction. As in the case of FIG. 13, in simultaneous erasingeffected in a unit of memory block, a memory block to be erasedsimultaneously is designated by the erase block designation register, sothat high voltage Vpp for erase is supplied to a source line of thedesignated memory block. Details of the voltage conditions for erase andwrite will be described later. YSEL designates a Y selection circuit, CDa common data line, WRIT a write circuit, DILAT a data input latch, SAMPa sense amplifier, DOLAT a data output latch, DIBUFF a data input bufferand DOBUFF a data output buffer.

[0238] The relation between memory array portions ARY0 to ARY7 andoutput data resembles that in FIG. 13. More particularly, one bit ofinput/output data corresponds to one memory mat. For example, data D0 isunder the charge of the memory array portion ARY0. By employing such aconfiguration of one memory mat per one input/output, the common dataline CD can be sectioned, one for each memory array portion, and neednot extend over a long distance throughout all of the memory arrayportions, as illustrated in FIGS. 25A and 25B. The length of the commondata line CD is much smaller than that of the common data line CD′.Accordingly, parasitic capacitance (Cst) associated with the common dataline CD can be reduced (Cst<<C′st) to contribute to speed-up of accessand a low voltage operation.

[0239] When memory blocks such as LMB and SMB are defined each in a unitof word line as shown in FIG. 24, a minimum memory block in the whole ofmemory array ARY having a parallel input/output bit number equal to onebyte has a storage capacity corresponding to that of one word line, thatis, the storage capacity corresponding to the number of memory cellsassociated with one word line over the entire memory array. This holdsregardless of the number of parallel input/output bits. Contrarily, whenmemory blocks are defined each in unit of data line as shown in FIG. 13,a minimum memory block in the whole of memory array has a storagecapacity complying with the number of parallel input/output bits toamount up to a storage capacity of 8 data lines (one data line isassociated with each memory mat). Accordingly, if the number of memorybits in the direction of data line is ⅛ of the number of memory bits inthe direction of word line, no difference takes place between memoryblocks defined in a unit of data line and memory blocks defined in aunit of word line. Practically, however, the number of memory bits inthe direction of data line is conditioned by the efficiency of layout ofsemiconductor integrated circuit formation or the efficiency ofaddressing memory cells, amounting up to about ½ of the number of memorybits in the direction of word line and in addition, due to the fact thatthe flash memory built in the microcomputer is connected to internaldata bus, the number of parallel input/output bits is defined in a unitof byte or word. For these reasons, the storage capacity of the minimummemory block can be reduced drastically in the case where memory blocksare defined each in a unit of word line. With the minimum size of memoryblock reduced, convenience of use of a memory block as a data area canbe improved further and besides the effect of preventing suchwastefulness that information is erased simultaneously together withinformation which need not substantially be rewritten and then thelatter information is again written can fulfill itself.

[0240] [14] Countermeasures Against Data Line Disturbance in anUnselected Block for Writing

[0241]FIGS. 26A and 26B show an example of the voltage conditions forerase/write available when memory blocks are defined each in a unit ofword line. Especially, countermeasures against data line disturbance areapplied to an unselected block (unselected memory block) for writing.

[0242] In FIG. 26A showing the voltage conditions for erase, a selectedblock (selected memory block) 20 is a memory block selected forsimultaneous erasing and an unselected block 21 is a memory block notselected for simultaneous erasing. In an erase operation, word lines WLhto WLk depicted as representatives are applied with ground potential GNDsuch as 0 V. In the selected block 20, its common source line SLwm isapplied with a high voltage Vpp of, for example, 12 V so that memorycells of the selected block 20 may be erased simultaneously. In theunselected block 21, its common source line SLwn is maintained at groundpotential GND to inhibit erase.

[0243] In FIG. 26B showing the voltage conditions for write, a selectedblock 30 is a memory block containing memory cells selected for writingand an unselected block 31 is a memory block not containing memory cellsto be written. In the selected block 30, a common source line SLwm isapplied with ground potential GND and when, for example, a memory cellMC circled by phantom line is to be written, high voltage Vpp is appliedto a word line WLh connected with its control gate and a relatively highvoltage Vp of, for example, 6 V is applied to its data line. In theselected block 30, a word line WLi not selected is applied with groundpotential GND.

[0244] Upon writing, in the unselected block 31, all word lines WLj andWLk are maintained at ground potential GND, so that memory cells arerendered not to be selected. Because of the nature of formation ofmemory blocks in a unit of word line, the data line extending into theunselected block 31 is also applied with voltage Vp in accordance withwrite effected in the selected block 30. Namely, a memory cell MC in theunselected block 31 is placed in the condition of word line unselectionand data line selection in accordance with write effected in theselected block 30. For example, in accordance with the condition shownin FIG. 26B, when a circled memory cell in the selected block is to bewritten, a memory cell (surrounded by a quadrangle of phantom line)inthe unselected block 31 and connected to a data line DLk associated withthe former memory cell is applied with voltage Vp. Then, a common sourceline SLwn in the unselected block 31 is applied with a voltage Vddi(data line disturbance prevention voltage) of, for example, 3.5 V toapply countermeasures against data line disturbance. If the source lineSLwn is applied with ground potential GND as in the case of the selectedblock 30, then data line disturbance will occur. In the selected block30, memory cells which are connected to data line DLk and are not to bewritten are applied with ground potential GND at their associated wordlines and source lines to set up the same condition as that responsiblefor occurrence of data line disturbance but such condition can besubstantially negligible. This will become apparent from item [15] of“Correlation of data line disturbance time between memory blocks” to bedescribed later with reference to FIG. 29.

[0245]FIG. 27A shows the mechanism of generation of data linedisturbance. More particularly, in a region (1) near the source side endof drain, electron and hole pairs are generated through a tunnelphenomenon between bands. At that time, if the source is maintained atground potential GND and,the drain is maintained at a relatively highvoltage Vp to generate a relatively large electric field, holes of theelectron and hole pairs are accelerated by an electric field in adepletion layer of the region (2) and turn into hot holes. The hot holespass through a thin tunnel insulating film of about 10 nm thickness(under floating gate electrode 8) so as to be injected into the floatinggate 8. This state is a data line disturbance state and when a memorycell transistor suffers from the data line disturbance for a long time,its threshold is decreased, with the result that the memory cell beingin write state “0” changes to erase state “1” and the memory cell beingin erase state “1” undergoes depletion to cause an unwanted change ofstored information or an eventual erroneous operation (data linedisturbance fault).

[0246]FIG. 27B shows the mechanism of countermeasurements against dataline disturbance. To describe, when potential on the source side israised in the unselected block for writing by applying a voltage Vddi of3.5 V to the source of a memory cell as shown in FIGS. 26A and 26B, anelectric field in a depletion layer as indicated by a region (2) isweakened and as a result, the turning of holes of electron and holepairs into hot holes is prevented to ensure that the reduction inthreshold of the memory cell transistor can be prevented.

[0247]FIG. 28 shows an example of experiment concerning the change ofthreshold of a memory cell with respect to data line disturbance time.In this experiment, a memory cell transistor as shown in the figure isused and write is repeated to obtain threshold voltages by maintainingthe source potential Vs at 0 V, floating (open) and 3.5 V, respectively,under the condition that ground potential GND is applied to the controlgate and substrate of the memory cell transistor and 6.5 V is applied tothe drain thereof. The upper portion of the figure is for the memorycell transistor being in the write state “0” and the lower portion isfor the memory cell transistor being in the erase state “1”. As is clearfrom the figure, for Vs=3.5 V, a decrease in threshold which is toolarge to be negligible does not occur within a data line disturbancetime of about 1000 seconds under any of the erase state and the writestate.

[0248] From the above, it will be understood that in order to preventoccurrence of faults due to data line disturbance, the source potentialof the unselected memory block needs to be biased with a data linedisturbance prevention voltage Vddi such as 3.5 V not higher than thedrain voltage and the data line disturbance time needs to be reduced toas small a value as possible.

[0249] [15] Correlation of Data Line Disturbance Time Between MemoryBlocks

[0250] The correlation of data line disturbance time between a memoryblock MBa having a relatively small storage capacity and a memory blockMBb having a relatively large storage capacity as shown in FIG. 29 willbe described. For convenience of explanation, the common source line ofan unselected block for writing is also maintained at ground potentialGND as in selected block for writing. The data line disturbance time inthis case is shown in Table 2. In this table, though not limitedly, thewrite time per memory cell one bit is set to 100 μsec. and the number oftimes of the erase and write is set to 10000. One erase and writeoperation referred to herein means such an operation that an objectmemory block is erased simultaneously and thereafter individual wordlines are sequentially selected to write memory cells. But the data linedisturbance time of a memory cell in the memory block selected forwriting is handled on the assumption that a word line to which thememory cell of interest is coupled is not selected. TABLE 2 data linedata line disturbance time disturbance time affecting MCa affecting MCbMBa selected, <PHASE A/A> <PHASE B/A> MBb unselected 100 μS × 15 × 1 100μS × 16 × 10⁴ time = 1.5 msec times = 1.6 sec MBb selected, <PHASE A/B><PHASE B/B> MBa unselected 100 μS × 1008 × 10⁴ 100 μS × 1007 × 1 times =1000 sec time = 0.1 sec

[0251] According to the results, the data line disturbance timeaffecting a memory cell MCa of the memory block MBa is 1.5 msec when thememory block MBa of interest is selected for writing (Phase A/A) and1000 sec. when the memory block MBb is selected (Phase A/B). Firstly,this difference is due to a difference in storage capacity (the numberof word lines) between the memory blocks MBa and MBb. Namely, this isdue to the fact that while in a calculation equation of data linedisturbance time shown in Phase A/A which is 100 μs×15×one frequency thetime of word line switching upon write following simultaneous erasing ofthe memory block is 15 which corresponds to the number of word lines ofthe memory block MBa, the number of times of word line switching uponwrite following simultaneous erasing of the memory block is 1008corresponding to the number of word lines of the memory block MBb in acalculation equation of data line disturbance time shown in Phase A/Bwhich is 100 μs×1008×10000 frequencies. Secondly, the above differenceis attributable to the fact that in calculation of the data linedisturbance time affecting the memory cell MCa in the memory block MBaselected for rewriting the number of times of substantial rewrite isdeemed as one. More specifically, the difference is due to the factwhile the number of times of rewrite is deemed as one in the calculationequation of data line disturbance time shown in Phase A/A which is 100μs×15×one time, the number of times of rewrite is 10000 which coincideswith the number of times of actual rewrite operations in the calculationequation of the data line disturbance time shown in Phase A/B which is100 μs×1008×10000 times. Presumably, this is because for the memory cellMCa in the memory block MBa selected for rewriting, threshold voltagesof all memory cells are raised through prewrite preceding simultaneouslyerasing and thereafter erase is carried out stepwise from the viewpointof prevention of excessive erase upon each rewrite operation asexplained with reference to FIG. 22, so that the data line disturbancetime of the memory cell MCa of interest is substantially defined by thetime for one rewrite operation. In other words, the data linedisturbance state affecting the memory cell MCa in the memory block MBaselected for rewriting is deemed as being initialized every rewriteoperation. Contrary to this, when a memory block selected for rewritingis the memory block MBb, the memory cell MCa does not undergo theinitialization and the data line disturbance time is accumulated inaccordance with the frequency of actual rewrite operations.

[0252] Similarly, the data line disturbance time affecting the memorycell MCb of the memory block MBb is 0.1 sec. when the memory block MBbof interest is selected as write object (Phase B/B) and is 16 sec. whenthe memory block MBa is selected (Phase B/A). As in the preceding, thisdifference is also due to the fact that the storage capacity (the numberof word lines) differs between the memory blocks and the substantialfrequency of rewrite operations is deemed as one in the calculation ofthe data line disturbance time affecting the memory cell MCb in thememory block MBb selected for rewriting.

[0253] From this, it is clear that the data line disturbance time towhich the unselected memory block is subjected owing to write of theselected memory block is far longer than the data line disturbance timeaffecting the memory cell in the selected memory block. Accordingly, itwill be appreciated that in order to prevent a decrease in thresholdvoltage of a memory cell due to data line disturbance, the common sourceline on the unselected memory block side for writing is at leastrequired to be biased by voltage Vddi but the data line disturbance timeaffecting the memory cell in the selected memory block can be neglectedwithout causing almost any troubles.

[0254] Further, the following will be clear from the contents of PhaseA/B and Phase B/A in the correlation of the data line disturbance timeshown in Table 2. Namely, the data line disturbance time (e.g., 1000sec) affecting an unselected memory block having a small storagecapacity owing to write of a memory block having a large capacity isrelatively larger than the data line disturbance time (e.g., 1.6 sec) inthe converse case.

[0255] [16] A Transfer Gate Circuit for Data Line Separation

[0256]FIGS. 30A and 30B show an embodiment of a memory array in which atransfer gate circuit for selectively separating data lines isinterposed between memory blocks. A transfer gate circuit TGC isarranged between memory blocks MBa and MBb and has transfer MOStransistors T0 to Tk associated with data lines DL0 to DLk in one to onecorrespondence relationship, the transfer MOS transistors beingcontrolled for switching by a control signal DT. According to thisexample, a Y selection circuit YSEL such as a column selection switchcircuit is arranged on the side of the memory block MBb. FIG. 30B showsswitch control modes of the transfer MOS transistors T0 to Tk. When thememory block MBa is a selected block for writing, the transfer MOStransistors T0 to Tk are rendered on. At that time, source potential Vsaof the memory block MBb serving as the selected block for writing ismaintained at ground potential GND and source potential Vsb of thememory block MBb standing for the unselected block for writing ismaintained at data line disturbance prevention voltage Vddi such as 3.5V. On the other hand, when the memory block MBb is a selected block forwriting, the transfer MOS transistors T0 to Tk are rendered off. At thattime, source potential Vsb of the memory block MBb standing for thewrite selected block is maintained at ground potential GND. Sourcepotential Vsa of the memory block MBa standing for an unselected blockfor writing may be maintained at either data line disturbance preventionvoltage Vddi such as 3.5 V or ground potential GND (or floating). Thisis because by virtue of the transfer MOS transistors T0 to Tk renderedto be cutoff, data line write voltage Vp fed through the Y selectioncircuit YSEL is not transmitted to the memory block MBa.

[0257] The transfer gate circuit TGC is in particular of significance inconnection with the data line disturbance time of an unselected blockfor writing as will be described below. More specifically, when thememory block MBa is set to be a selected block for writing, a relativelyhigh voltage Vp for write of the memory block MBa is applied through adata line to the memory block MBb preceding the transfer gate circuitTGC (on the side of Y selection circuit YSEL). Under this condition, thecommon source line of the memory block MBb serving as an unselectedblock for writing is applied with data line disturbance preventionvoltage Vddi to essentially prevent data line disturbance but as thiscondition continues for a long time (resulting in considerableprolongation of the data line disturbance time), the threshold of amemory cell subject to write condition in the unselected memory blockfor writing MBb slightly decreases even if the source of the memory cellis biased by voltage Vddi, as will be clear from FIG. 28. Thus, bytaking advantage of the fact that the data line disturbance timeaffecting a memory block of a small storage capacity owing to writeconcomitant with rewrite of a memory block of a large storage capacityis relatively larger than the data line disturbance time in the conversecase, as described with reference to FIGS. 30A and 30B, the transfergate circuit TGC intervenes such that the memory block MBb on the sideof Y selection circuit YSEL is made to be a large memory block having arelatively large storage capacity and the memory block MBa on theopposite side is made to be a small memory block having a relativelysmall storage capacity. By making the memory block MBa a small memoryblock and making the memory block MBb a large memory block in thismanner, the data line disturbance time affecting a memory cell of thememory block MBb owing to write of the memory block MBa can be farshorter than the data line disturbance time in the case where the memoryblock MBa is made to be a large memory block and the memory block MBb ismade to be a small memory block. Through this, prevention of anerroneous operation due to data line disturbance can further beperfected.

[0258] The countermeasures against data line disturbance are summed upin FIG. 31. In the figure, the voltage application condition showingcountermeasures against data line disturbance for the unselected memoryblock shown at (A) is representative of a memory cell transistorconnected to a data line which is interrupted from the supply of writevoltage by the off state of the transfer gate circuit TGC.

[0259] [17] Dummy Word Lines

[0260]FIGS. 32, 33 and 34 are circuit diagrams in which a dummy wordline is arranged between a memory block and a transfer gate circuit. Ineach of the figures, DWA designates a dummy word line on the side of amemory block MBa, and DWB a dummy word line on the side of a memoryblock MBb. One DWA of the dummy word lines is coupled with control gatesof dummy cells DC1 to DC3 and the other DWB is coupled with controlgates of dummy cells DC4 to DC6. Each of the dummy cells DC0 to DC6 isformed of the same transistor as that of a memory cell. In FIG. 32, thedummy cells DC0 to DC6 have their sources made to be floating and theirdrains coupled to data lines. In FIG. 34, the dummy cells DC0 to DC6have their sources and drains which are made to be floating. In FIG. 34,the dummy cells DC0 to DC6 have their sources connected to a commonsource line of a corresponding memory block and their drains made to befloating. When a transfer gate circuit TGC is provided between memoryblocks, a repetitive pattern of memory cell transistors and word linesis interrupted at a position of the transfer gate circuit and from thestandpoint of device structure, there results an abrupt unevenness inthe wafer surface. Such an unevenness leads to non-uniformity ofthickness of a photoresist film formed when word lines and control gatesare formed through, for example, photoetching. This causes partialnon-uniformity of dimensions of word lines and control gates andirregularity in electrical characteristics of transistors and word linesresults. Under the circumstances, by arranging the dummy word lines DWAand DWB and the dummy cells DC0 to DC3 and DC4 to DC6 at respective endsof the memory blocks MBa and MBb separated by the transfer gate circuitTGC, irregularity in dimensions of word lines and control gates whichoccurs near the transfer gate circuit TGC can be reduced.

[0261] [18] Various Forms of Pluralization of Memory Blocks in a Unit ofWord Line

[0262] As shown in FIG. 35, two memory blocks can be arranged on eachside of a transfer gate circuit TGC. Preferably, in this case, memoryblocks MBc and MBd on the side of a Y selection circuit YAEL are made tobe large memory blocks and memory blocks MBb and MBa succeeding thetransfer gate circuit TGC are made to be small memory blocks. Forexample, the large memory block is used for program storage and thesmall memory block is used for data storage.

[0263] As shown in FIG. 36, a simultaneously erasable, minimum memoryblock has a single word line and the number of word lines can beincreased sequentially to two, three and four. But the number of wordlines of individual, simultaneously erasable memory blocks may bedetermined suitably or the size of individual memory blocks may bechanged suitably.

[0264] As shown in FIG. 37, when a group of relatively small memoryblocks MBa to MBe respectively having one, two, three, four and eightword lines and a group of relatively large memory blocks MBf each having64 word lines are employed, a transfer gate circuit TGC may preferablybe arranged at a boundary part between the large and small memory blockgroups as will be inferred from the explanation of the previous item[16].

[0265] As shown in FIG. 38, the data line structure is constructed ofmain data lines and subsidiary data lines. Main data lines DL0 to DLkextend to reach all memory blocks MBa to MBc. Subsidiary data lines d0to dk extend only inside each memory block to connect to drains ofmemory cells contained in a corresponding memory block. In this case,connection of the main data lines DL0 to DLk to the subsidiary datalines d0 to dk is set up through a transfer gate circuit TGC allotted toeach memory block. Such a structure can be realized easily by, forexample, a two-layer aluminum wiring structure. Since in themain/subsidiary data line structure the transfer gate circuit TGC isprovided in each memory block, write data line potential Vp can beapplied to only a selected block for writing. Accordingly, countermeasurements against data line disturbance can further be perfected.

[0266]FIG. 39 shows an embodiment in which simultaneously erasablememory blocks are arranged on the left and right sides of an X addressdecoder. Decode signals of X address decoder XADEC are delivered to theleft and right sides thereof. Then memory blocks MBa to MBc and MBa ′toMBc′ each defined in a unit of word line arranged on each side of the Xaddress decoder XADEC are provided on the left and right sides thereof.As each memory block, any one of the previously described memory blockscan be adopted. The memory blocks on the left and right sides performinput/output of data io0 to io7 and data io8 to io15, respectively, in aunit of 8 bits through Y selection circuits YSEL and YSEL′. Transfer MOStransistors Tsw are provided between left-hand outputs of the X addressdecoder XADEC and word lines WL0 to WLn in one to one correspondencerelationship and similarly, transfer MOS transistors Tsw′ are providedbetween right-hand outputs of the X address decoder XADEC and word linesWLO ′to WLn′ in one to one correspondence relationship. Further, theleft-hand word lines are associated with discharge MOS transistors Cswand the right-hand word lines are associated with discharge MOStransistors Csw′. A control circuit DIVCONT is, responsive to a highvoltage Vppl (=Vpp such as 12V) and the most significant address bit An,adapted to perform switching control of the left-hand transfer MOStransistors Tsw and discharge MOS transistors Csw and of the right-handtransfer MOS transistors Tsw′ and discharge MOS transistors Csw′. Thoughnot limitedly, the control circuit DIVCONT receives a high voltage Vppland the most significant address bit An of an address signal andperforms complementary switching control between the left-hand transferMOS transistors Tsw and discharge MOS transistors Csw and the right-handtransfer MOS transistors Tsw′ and discharge MOS transistors Csw′ inaccordance with a logical value of the most significant address bit An.For example, when the most significant address bit An is logical “1”,the right-hand transfer MOS transistors Tsw′ are rendered to be on andthe left-hand transfer MOS transistors Tsw are rendered to be off, sothat write data can be supplied through the right-hand Y selectioncircuit YSEL′. At that time, the right-hand discharge MOS transistorsCsw′ are rendered to be off and the left-hand discharge MOS transistorsCsw are rendered to be off. When the most significant address bit An islogical “0”, the left-hand transfer MOS transistors Tsw are rendered tobe on and the right-hand transfer MOS transistors Tsw′ are rendered tobe off, so that write data is supplied through the left-and Y selectioncircuit YSEL. At that time, the right-hand discharge MOS transistorsCsw′ are rendered to be on and the left-hand discharge MOS transistorsCsw are rendered to be off. Selection operation of the left-hand andright-hand Y selection circuits YSEL and YSEL′ depends on a decodeoutput of a Y address decoder YADEC but any one of the left-hand andright-hand Y selection circuits YSEL and YSEL′ may be activated by themost significant address bit An or a signal equivalent thereto or anyone of the left-hand and right-hand Y selection circuits may be selectedas the supply path of write data by means of a separate selectioncircuit not shown. A signal voltage for rendering the transfer MOStransistors Tsw and Tsw′ on is set to a high voltage during write and anexample of the control circuit DIVCONT for this purpose is shown in FIG.40. A voltage Vppl in FIG. 40 can be generated using a power supplycircuit of FIG. 51 to be described later.

[0267] As a comparableness to the configuration shown in FIG. 39, aconfiguration may be mentioned in which an X address decoder is arrangedat one end side of word lines. In this case, the word line directionsize of a memory block defined in a minimum unit of word line is doubledas compared to that in FIG. 39. In comparison with the configuration ofFIG. 39, this configuration can contribute to reduction of the word linedisturbance time of a selected block for writing. More particularly,with reference to FIG. 26B, in the selected block 30 for writing, thereis a memory cell associated with a word line applied with high voltageVpp and with a data line not applied with write voltage Vp. In theselected block 30 for writing, the memory cell placed in word lineselection condition and data line non-selection condition suffers from alarge potential difference between the control gate and floating gate,with the result that electric charge is discharged from the floatinggate to the control gate and the threshold of the memory cell transistoris forced to be decreased undesirably. This phenomenon is word linedisturbance and in proportion to prolongation of this state, thethreshold decreases. Accordingly, like the data line disturbance, timefor the word line disturbance state to continue (word line disturbancetime) is desired to be short. From this viewpoint, the configuration ofFIG. 39 is more advantageous than the comparative configuration in thatthe number of memory cells exposed to the word line disturbance statecan be halved on the presumption that write is effected in a unit of 8bits. This contributes to reduction of the word line disturbance time.

[0268]FIG. 41 shows an embodiment directed to the provision of redundantwords in a memory block. In the figure, a redundant word line WRa, aredundant data line DR and redundant memory cells RC for relieving adefective word line are arranged in a memory block MBa and a redundantword line WRb, a redundant data line DR and redundant memory cells RCfor the same purpose are arranged in a memory block MBb. With theredundant words provided in the memory blocks MBa and MBb, when adefective word is desired to be relieved, the defective word can berelieved using a redundant word in the same block as a memory block towhich the defective word belongs. For example, in the event that a wordin the memory block MBa is defective, the word can be relieved by aredundant word WRa in the memory block MBa of interest. Through this,even when the defective word is replaced with the redundant word, thecounter measurements against data line disturbance can also be appliedto the redundant word under quite the same condition. As redundantwords, memory blocks MBrd and MBrd′ dedicated to redundancy may beprovided as shown in FIG. 42.

[0269]FIG. 43 shows an embodiment in which some memory blocks are formedinto one-time programmable areas (OTP-flash). In an area formed into aone-time programmable area, only one-time write of desired data isallowed. In the figure, memory blocks MBc and MBd are memory blockswhich are formed into one-time programmable areas. Structurally, thememory blocks MBc and MBd per se which are formed into one-timeprogrammable areas are quite the same as the other memory blocks. Aspecified memory block can be formed into a one-time programmable areaby selectively suppressing rewrite of the memory block in question. Forexample, a designation bit of an erase register for designating a memoryblock which is scheduled to be formed into a one-time programmable areais forced to assume an unselection level by means of a nonvolatilememory device and a path for supplying a write voltage to a word line ofthe memory block of interest is so designed as to be breakable by meansof the nonvolatile memory device. Through this, the memory block formedinto a one-time programmable area and the other memory blocks can havein common an X address decoder, a Y address decoder and data lines. Inthis case, most conveniently, a transistor similar to a memory celltransistor of the flash memory may be utilized as the nonvolatile memorydevice. In a write operation, source lines Vsc and Vsd of the memoryblocks formed into one-time programmable areas are applied with the dataline disturbance prevention voltage Vddi to prevent a data linedisturbance defect of these memory blocks. With some memory blocksformed into one-time programmable areas in this manner, occurrence ofsuch a trouble that data once written is subsequently rewrittenundesirably can be prevented. For example, the memory block formed intoa one-time programmable area can be utilized as a program holding areaor a data holding area which needs to be prevented from being altered.

[0270]FIG. 44 shows a configuration in which some memory blocks areformed into mask ROM's, in place of the configuration in which somememory blocks are formed into one-time programmable areas. In thefigure, memory blocks MBc and MBd are areas which are formed into maskROM's. By adopting this configuration, write of the memory blocks MBcand MBd becomes absolutely impossible. Upon write, word lines of thememory blocks MBc and MBd formed into mask ROM's are inhibited frombeing applied with a write high voltage and source lines Vsc and Vsd ofthese memory blocks are biased by a voltage such as Vddi. Upon erase,the common source lines Vsc and Vsd of the memory blocks MBc and MBd areinhibited from being applied with an erase high voltage.

[0271] [19] Layout Configuration of Memory Blocks

[0272]FIG. 45 shows an example of a layout configuration of memoryblocks. The layout configuration shown in the figure is an example wherea transfer gate circuit TGC is not arranged between memory blocks MBaand MBb. In the figure, a memory cell includes a control gate 11integral with a word line, a floating gate (fg) 8 formed separately fromthe control gate to underlie it, a drain formed of an N typesemiconductor region 13 and a P type semiconductor region 14, and asource formed of the N type semiconductor region 13 and an N typesemiconductor region 15. Individual memory cells are separated from eachother by a thick field insulating film 4. Word lines WLO to WLi+2 areseparated from each other and formed in parallel in the lateraldirection in the figure. Data lines DL0 to DL8 are formed of a firstwiring layer 23 having a first aluminum layer (All) and are separatedfrom each other so as to be arranged in intersectional relation with theword lines, thus extending in parallel in the longitudinal direction inthe figure. The data line is connected to the drain common to anadjacent memory cell through a contact (CONT) 22. Sources of memorycells are each formed of the N type semiconductor regions 13 and 15 andare connected every 8 bits to a source line SL formed of the firstwiring layer 23 through the contact 22. The source line SL is parallelto the data lines DL0 to DL8. The source line SL in each memory block isbroken at the block end so as to be disconnected from the source line SLof an adjacent memory block. Contrarily to this, the data lines DL0 toDL8 extend to pass through adjacent blocks. The source line SL in onememory block connects, at the block end, to a common source line SA orSB formed of a second wiring layer (A12) such as a second aluminum layerthrough a through hole (TC) 25. The common source lines SA and SB extendin parallel to the word lines to underlie the field oxide film 4. Inthis manner, the source line in a unit of memory block is providedseparately. The common source line SA or SB may be arranged at oppositeblock ends or at the center of the memory block. Though not illustrated,each word line is shunted every 16 bits to the second wiring layer 26overlying the word line to reduce a delay component of the word line.

[0273]FIG. 46 shows an example of a layout configuration in which atransfer gate circuit is provided between memory blocks. The transfergate circuit is constructed by arranging transfer MOS transistors TO toT8 each formed of a high breakdown voltage N channel type MOS transistorhaving its gate electrode in the form of a first conductive layer 8between common source lines SA and SB of adjacent memory blocks MBa andMBb. In this case, data lines are broken at adjacent ends of the memoryblocks MBa and MBb. A broken end of one of the data lines havingmutually opposing broken ends is connected to the drain of each of thetransfer MOS transistor T0 to T8 through a contact 22 and a broken endof the other data line is connected to the source of each of thetransfer MOS transistors T0 to T7 through a contact 22. A memory cell atan end of each of the opposing memory blocks is used as a dummy cellhaving its source being, in this example, floating. FIG. 47 shows aconfiguration in which the drain of a dummy cell is floating in contrastto the configuration of FIG. 46.

[0274]FIG. 48 shows an example of a layout configuration in whichtransfer MOS transistors T0 to T7 are substantially increased in size.In this example, the gate width of each of the transfer MOS transistorsT0 to T7 is increased to prevent a decrease in data line potentialcaused by each of the transfer MOS transistors T0 to T7. Morespecifically, in the example of FIG. 48, the transfer MOS transistorsT0, T2, T4 and T6 are arranged on the side of a memory block MBa inparallel with word lines and the transfer MOS transistors T1, T3, T5 andT7 are arranged on the side of a memory block MBb in parallel with wordlines. A data line DL0 extending from the side of memory block MBbpasses above the transfer MOS transistor T1 so as to be coupled to thetransfer MOS transistor T0 and a data line DL0 extending from the sideof memory block MBa is coupled to the transfer MOS transistor T0. Anadjacent data line DL1 extending from the side of memory block MBapasses above the transfer MOS transistor T0 so as to be coupled to thetransfer MOS transistor T1 and a data line DL1 extending from the sideof memory block MBb is coupled to the transfer MOS transistor T1. Theother transfer MOS transistors are also coupled to data lines in asimilar manner. The number of stacking of transfer MOS transistors isnot limited to 2 as above but can equal the number of data lines betweensource lines SL at maximum.

[0275] [20] The Whole of a Flash Memory Applied with CountermeasuresAgainst Data Line Disturbance

[0276]FIG. 49 is a block diagram showing an embodiment of the whole of aflash memory applied with pluralization of memory blocks in a unit ofword line and countermeasures against data line disturbance. The flashmemory shown in the figure is incorporated in a microcomputer. In thefigure, 210 designates a memory array in which memory cells eachconstructed of an insulated gate field effect transistor of two-layergate structure as previously explained with reference to, for example,FIG. 11 are arranged in matrix. In the memory array ARY, as in the caseof the configuration explained in connection with FIG. 25, memory cellshave control gates connected to corresponding word lines, drain regionsconnected to corresponding data lines and source regions connected tosource lines SL1 to SLn being each common to respective memory blocksMB1 to MBn defined each in a unit of word line. The source lines SL1 toSLn of the respective memory blocks are separately connected to erasecircuits ERS1 to ERSn, respectively. In the figure, n memory blocks MB1to MBn are shown and as exemplified in FIG. 18, these memory blocks maybe divided into 7 large memory blocks (large blocks) LMB0 to LMB6 eachhaving a relatively large storage capacity and 8 small memory blocks(small blocks) SMB0 to SMB7 each having a relatively small storagecapacity. The large memory block may be used as a program storing areaor a large capacity data storing area. The small memory block may beused as a small capacity data storing area.

[0277] In FIG. 49, 200 designates an address buffer and address latchcircuit having its input coupled to an internal address bus of amicrocomputer. Denoted by 201 is an X address decoder (XADEC) fordriving a word line by decoding a row address signal (X address signal)latched in the address buffer and address latch circuit 200. Forexample, the X address decoder 201 drives a given word line with avoltage of, for example, 5V in a data read operation and drives a givenword line with a high voltage of, for example, 12V in a data writeoperation. In a data erase operation, all outputs of the X addressdecoder 201 are maintained at a low voltage level of, for example, 0V.Denoted by 202 is a Y address decoder (YADEC) for decoding a Y addresssignal latched in the address buffer and address latch circuit 200.Denoted by 203 is a Y selection circuit (YSEL) for selecting a data linein accordance with a data line selection signal delivered out of the Yaddress decoder 202. The data line is related to the Y selection circuitsuch that one memory mat corresponds to one I/O as described withreference to FIG. 25. Though not limitedly, the memory array is dividedinto 16 memory mats. In this case, each of the memory blocks MB1 to MBnextends over 16 memory mats. Denoted by 204 is a sense amplifier (SAMP)for amplifying a read signal from a data line selected by the Yselection circuit 203 in a data read operation. According to the presentembodiment, the sense amplifier includes 16 amplifier circuits incorrespondence to output bits of the memory mats. Denoted by 205 is adata output latch (DOLAT) for holding an output of the sense amplifier204. Denoted by 206 is a data output buffer (DOBUFF) for delivering dataheld in the data output latch 205 to the outside. An output of the dataoutput buffer 206 is coupled to a 16-bit internal data bus of themicrocomputer in bit correspondence relationship. According to thisexample, read data is of 2 bytes at maximum. Denoted by 207 is a datainput buffer (DIBUFF)for fetching write data supplied from the outside.Data fetched in from the data input buffer 207 is held in a data inputlatch (DILAT) 208. When the data held in the data input latch 208 is“0”, a write circuit (WRIT) 209 supplies a write high voltage to a dataline selected by the Y selection circuit 203. This write high voltage issupplied to the drain of a memory cell having its control gate appliedwith a high voltage in accordance with an X address signal, causing thememory cell of interest to be written.

[0278] Each of the erase circuits ERS1 to ERSn supplies an erase highvoltage to a source line of a designated memory block to performsimultaneous erase of the memory block. Which one of the erase circuitsis to be caused to effect an erase operation is controlled by a settingbit of an erase block designation register 231. The erase blockdesignation register 231 corresponds to the registers MBREG1 and MBREG2explained with reference to FIG. 18. As described with reference to FIG.26, the erase circuits ERS1 to ERSn are operative upon writing to applyground potential GND to a source line of a selected block for writingand data line disturbance prevention voltage Vddi to a source line of anunselected block for writing. This control is carried out by anunselected block designation circuit for writing 230. The unselectedblock designation circuit for writing 230 receives an X address signaldelivered out of the address buffer and address latch circuit 200 anddecodes the signal to decide a selected block for writing, designate theapplication of ground potential GND to an erase circuit associated withthe selected block for writing and designate the application of dataline disturbance prevention voltage Vddi to an erase circuit associatedwith an unselected block for writing.

[0279] In FIG. 49, 240 designates a control circuit for performingtiming control of a data read operation and selection control of varioustimings and voltages for write and erase.

[0280]FIG. 50 shows an example of the control circuit 240. The controlcircuit 240 has a power supply circuit 241, a memory read/write controlcircuit 242, a register control circuit 243 and a control register 244.The control register 244 has the program/erase control register PEREGexplained in connection with FIGS. 16 and 18. The control circuit FCONTshown in FIG. 16 may be considered to correspond to the control circuit240 and the erase block designation register 231 shown in FIG. 49. Anerase signal E, a write signal W, an erase verification signal EV and awrite verification signal WV correspond to an E bit, a P bit, an EV bitand a PV bit of the program/erase control register PEREG. As describedwith reference to FIG. 18, an erase/write operation is controlled inaccordance with the contents set in the program/erase control registerPEREG. On the basis of a read/write signal R/W1 and the like suppliedthrough a control bus, the register control circuit 243 controlsread/write of the program/erase control register PEREG and erase blockdesignation register 231 (MBREG1 and MBREG2) included in the controlregister 244. On the basis of a read/write signal R/w2 and the likesupplied through the control bus, the memory read/write control circuit242 controls operation of the data input buffer 207, data input latchcircuit 208, data output buffer 206, data output latch circuit 205,address buffer and address latch circuit 200 as well as operation of thepower supply circuit 241. The power supply circuit 241 receives powersupply voltage Vcc such as 5V and high voltage Vpp such as 12V and formsvoltages Vppl, VppS and Vccl in accordance with a setting bit of theprogram/erase control register PEREG included in the control register244 and an output control signal of the memory read/write controlcircuit 242.

[0281]FIG. 51 shows an example of a circuit diagram of the power supplycircuit 241. The power supply circuit 241 includes a reference voltagegeneration circuit 2410, a decoder drive power supply circuit 2411, asource circuit drive power supply circuit 2412 and a sense amplifierdrive power supply circuit 2413. In the reference voltage generationcircuit 2410, a high voltage Vpp (e.g., 12V) is divided by resistors togenerate reference voltages V1 (e.g., 3.5V) and V2 (e.g., 6.5V). Inaccordance with the operation state of the flash memory the decoderdrive power supply circuit 2411 generates voltage Vppl for determining adrive voltage of word line. The operation state of the flash memory istransmitted to the power supply circuit 241 by means of a control signal2414 from the control register 244 and memory read/write control circuit242 so that an internal switch circuit may be controlled to optimize thevalue of voltage Vppl in accordance with the operation state. An exampleof an output waveform of voltage Vppl varying with the internaloperation state is shown in FIG. 52. The decoder drive power supplycircuit 2411 has a detection circuit 2415 for detecting ordiscriminating whether the power supply voltage Vcc has a higher voltage(e.g., 5V) or a lower voltage (e.g., 3V) than the threshold voltage(e.g., 4V) and a booster circuit 2416 for boosting power supply voltageVcc when the power supply voltage Vcc is detected as being lower thanthe threshold voltage. A boosted voltage is utilized when a readoperation is effected with the power source voltage Vcc (such as of 3V)lower than the threshold voltage. The source circuit drive power supplycircuit 2412 generates voltage VppS utilized for drive of source line inaccordance with the control signal 2414. The sense amplifier drive powersupply circuit 2413 generates voltage Vccl utilized as drive voltage ofthe sense amplifier in accordance with the control signal 2414. Voltagewaveforms of voltages VppS and Vccl varying with the internal state ofthe flash memory are depicted in FIG. 52.

[0282]FIG. 53A shows an example of the X address decoder 201. In thefigure, the construction corresponding to one word line is typicallyillustrated. The X address decoder consists of a pre-decoder 2010 forreceiving an X address signal, a decode section 2011 for decoding anoutput of the pre-decoder and a drive section 2011 for driving a wordline on the basis of an output of the decode section 2011. Thepre-decoder 2010 and decode section 2011 are operated with a powersupply voltage Vcc such as 5V system. The drive section 2012 isconstructed as a high voltage driven system which is driven by a voltagesuch as the voltage Vppl. Denoted by 2013 is a high breakdown voltage Nchannel type MOS transistor for separating the 5V system from the highvoltage system.

[0283] When the transfer gate circuit TGC as described with reference toFIGS. 32 to 35 is adopted, the large memory blocks LMB0 to LMB6 shown inFIG. 16 correspond to memory blocks MB1 to MB7 of FIG. 49 and the smallmemory blocks SMB0 to SMB7 correspond to memory blocks MB8 to MBn ofFIG. 49. In FIG. 49, the transfer gate circuit TGC is arranged betweenthe memory blocks MB7 and MB8, though not illustrated particularly. FIG.53B shows an example of a selection circuit 250 for generating aswitching signal DT of the transfer gate circuit TGC. The selectioncircuit 250 receives the voltage Vppl from the power supply circuit 241,address signal from the address buffer 20B and write signal from thecontrol circuit 240 to cut off the transfer gate circuit TGC upon writeof the large memory blocks. In particular, the signal DT is set to 0Vcorresponding to ground potential upon write of the large memory blockbut in the other case, set to voltage Vppl, though not limitedly.

[0284]FIG. 54 shows an example of the erase circuit and FIG. 55 showsits operational timing chart. Supplied to each of the erase circuitsERS1 to ERSn is operating voltage represented by the voltage VppS andpower supply voltage Vdd. Signal E/W^(★) shown in the figure is a signalwhich is maintained at 0 level upon write or erase. When a bit suppliedfrom the erase block designation register to the erase circuit of FIG.54 is “1” level (erase designating level), erase signal E from thecontrol circuit 240 is also rendered to be “1” level and supply voltageVs to a source line is set to the voltage VppS. Voltage Vpps upon eraseis set to Vpp as described with reference to FIG. 52. Through this, in aselected block for simultaneous erasing, simultaneous erase of memorycells can be done. When a control signal supplied from the upon-writeunselected block designation circuit to the erase circuit of FIG. 54 is“1” level (level for designating an unselected block) for writing, writesignal W from the control circuit 240 is also rendered to be “1” leveland supply voltage Vs to the source line is set to the voltage VppS.Voltage VppS upon write is maintained at data line disturbanceprevention voltage Vddi such as 3.5V. Through this, data linedisturbance can be prevented in the unselected block for writing.

[0285]FIG. 56 shows a timing chart of a series of operations related toerase in the flash memory shown in FIG. 49 and FIG. 57 shows a timingchart of a series of operations related to write in the flash memoryshown in FIG. 49. Prior to giving a description of each timing chart,control signals shown in these figures will first be described. Part ofthe contents of the description given in connection with FIG. 16 will berepeated herein because this is considered to be necessary forfacilitating understanding. Control signal FLM is a signal fordesignating the operation mode of the flash memory FMRY, whereby its “0”designates the first operation mode and its “1” designates the secondoperation mode. This signal FLM is formed on the basis of, for example,the mode signals MD0 to MD2. Control signal MS-MiSN is a selectionsignal of the flash memory FMRY, whereby its “0” designates selectionand its “1” designates unselection. Control signal MS-MISN is aselection signal of internal registers such as the program/erase controlregister PEREG and erase block designation registers MBREG1 and MBREG2.Which one of the registers is to be selected is determined by an addresssignal PABm. Denoted by M2RDN is a memory read strobe signal, by M2WRNis a memory write strobe signal, by MRDN is a read signal of registersbuilt in the flash memory and MWRN is a write signal of registers builtin the flash memory The memory write strobe signal M2WRN is deemed as astrobe signal for writing data to be written in a memory cell into thedata input latch DILAT. Actual write to the memory cell is started bysetting a P bit of the program/erase control register PEREG.

[0286] A series of operations related to erase are mainly sorted into asetup erase, an erase and an erase verify as shown in FIG. 56. The setuperase consists of an operation of writing data for designation of amemory block to be erased simultaneously into the erase blockdesignation register and an operation of writing a bit (flag) of logic“1” into an E bit of the program/erase control register PEREG. The eraseis an operation of erasing a memory block simultaneously and is startedby setting “1” in the E bit. The specific processing procedure of anerase operation is the same as the contents explained in connection withFIG. 22. The erase verify is started by clearing the E bit so thatverify may be carried out sequentially in a unit of byte, beginning witha head address, in accordance with the contents explained in connectionwith FIG. 22.

[0287] As shown in FIG. 57, a series of operations related to write aremainly sorted into a setup program, a program and a program verify. Thesetup program consists of an operation of writing data to be writteninto the data input latch circuit, an operation of storing a memoryaddress to be written to the address buffer and address latch circuitand an operation of writing a bit (flag) of logic“1” to a P bit of theprogram/erase control register PEREG. The program is an operation ofwriting a memory cell designated by a latched address in accordance withdata written in the data input latch circuit. A specific processingprocedure of a write operation is the same as the contents explained inconnection with FIG. 22. The program verify is started by clearing the Pbit so that verify may be carried out sequentially in a unit of byte,beginning with a head address, in accordance with the contents explainedin connection with FIG. 22.

[0288] Operation timings shown in FIGS. 57 and 58 are essentially thesame for any of the first and second operation modes and techniquesdescribed in the foregoing items [3] and [4] can be adopted. Whenrewrite is conducted by means of the general purpose PROM writer, partof processings can be put under the charge of the CPU built in themicrocomputer and other logics by utilizing a rewrite support controlprogram precedently prepared in the mask ROM built in the microcomputer.The flash memory shown in FIG. 49 can obviously be applied to themicrocomputer MCU explained in connection with FIGS. 1 to 4 or may beconstructed as a unitary flash memory chip.

[0289] [21] A Method for Production of a Flash Memory

[0290]FIGS. 58A to 58I show longitudinal sectional views of a device inthe production processes of various transistors for constituting theflash memory or the microcomputer incorporating the same. Illustrated ineach figure are six kinds of transistors which are, as viewed from theleft in turn in each figure, a memory cell transistor of the flashmemory, high breakdown voltage NMOS and PMOS used for write and erase ofthe flash memory, logic system NMOS and PMOS for formation of aperipheral logic such as CPU and a Zener diode used for generation of areference voltage upon write/erase read of the flash memory.

[0291] (A) Process Shown in FIG. 58A

[0292] (1) N type wells 2 and P type wells 3 are formed in a majorsurface of a P type semiconductor substrate 1 through known techniques.

[0293] (B) Process Shown in FIG. 58B

[0294] (1) P type channel stopper layers 5 are formed throughsubstantially the same process as that of thick field insulating films 4through known techniques.

[0295] (2) Then first gate insulating films 6 of the high breakdownvoltage NMOS (N channel type MOS transistor) and PMOS (P channel typeMOS transistor) are formed. The gate insulating film 6 is so formed asto have a thickness of 30 to 50 nm by a thermal oxidization methodconducted at a temperature of 850 to 950° C.

[0296] (C) Process Shown in FIG. 58C

[0297] (1) The first gate insulating film 6 is removed at an area forformation of the flash memory by using a mask such as photoresist toexpose the surface of the P type semiconductor substrate 1.

[0298] (2) Mask materials such as photoresist are removed.

[0299] (D) Process Shown in FIG. 58D

[0300] (1) Insulating films of about 10nm are formed (not shown) by athermal oxidization method conducted at a temperature of 800 to 850° C.

[0301] (2) Then the insulating films described in (1) are removedthrough wet etching. Through this, contaminants which are deposited onor which intrude into the exposed surface portion of P typesemiconductor substrate 1 at the area for flash memory formation whenremoving the mask such as photoresist in (1) of the above (C) can beremoved.

[0302] (3) A tunnel insulating film 7 of the flash memory is newlyformed. The tunnel insulating film 7 is so formed as to have a thicknessof 8 to 12 nm by a thermal oxidization method conducted at a temperatureof 800 to 850° C. At that time, the first gate insulating films 6 gothrough processes of (1) to (3) of the aforementioned (D) to have a filmthickness of 20 to 40 nm.

[0303] (4) Subsequently, first conductive layers 8 are formed whichserve as a floating gate electrode of the flash memory and gateelectrodes of the high breakdown voltage NMOS and PMOS. The firstconductive layer 8 is formed by diffusing phosphorus, through thermaldiffusion, in polycrystalline silicon deposited to a film thickness ofabout 200 nm at a temperature of about 640° to provide a sheetresistance ρs=60 to 100 Ω/□. In order to reduce irregularity in erase ofthe flash memory, the grain size of polycrystalline silicon needs to besmall and therefore the thermal diffusion is conducted at a temperatureof 900° C. or less to provide a grain size of 0.1 μm or less.

[0304] (E) Process Shown in FIG. 58E

[0305] (1) An inter-layer insulating film 9 is formed between a floatinggate electrode and a control gate electrode of the flash memory. Theinter-layer insulating film 9 is a laminated film of a silicon oxidefilm and a silicon nitride film, the laminated film consisting of, asviewed from the first conductive layer 8 side, a two-layer film ofsilicon oxide film and silicon nitride film or a four-layer film ofsilicon oxide film, silicon nitride film, silicon oxide film and siliconnitride film. Here, the silicon oxide film overlying the firstconductive layer 8 is formed to have a film thickness of 10 to 20 nmthrough thermal diffusion conducted at a temperature of 850 to 950°. Thesilicon nitride film overlying the silicon oxide film is formed to havea film thickness of 20 to 30 nm through CVD process. In the case of thefour-layer film, the silicon oxide film on the silicon nitride film areformed to have a film thickness of 2 to 5 nm by a thermal oxidizationmethod conducted at a temperature of 900 to 950°. The silicon nitridefilm overlying the silicon oxide film of 2 to 5 nm is formed to have athickness of 10 to 15 nm through CVD process. Either of the two-layerfilm and four-layer film is so formed as to have a total thickness of 20to 30 nm in terms of silicon oxide film.

[0306] (2) The inter-layer insulating films 9 at areas for formation ofthe logic system NMOS and PMOS as well as the Zener diode are removedusing a mask such as photoresist.

[0307] (3) The mask such as photoresist is removed.

[0308] (4) The first gate insulating films 6 at areas for formation ofthe logic system NMOS and PMOS as well as the Zener diode are removedthrough wet etching using the uppermost silicon nitride film of theinter-layer insulating film 9 as a mask, thus exposing the surface ofthe P type semiconductor substrate 1.

[0309] (F) Process Shown in FIG. 58F

[0310] (1) Contaminants deposited on or intruding into the exposedportion of surface are removed through similar techniques in (1) and (2)of the above (D). At that time, an insulating film of 10 to 20 nm isformed by a thermal oxidization method conducted at 800 to 850° C.

[0311] (2) Then, second gate insulating films 10 serving as gateinsulating films of the logic system NMOS and PMOS are formed. Thesecond gate insulating film 10 is formed to have a thickness of 10 to 20nm in wet atmosphere by a thermal oxidization method conducted at 800 to850° C.

[0312] (3) Subsequently, second conductive layers 11 are formed whichserve as a control gate electrode of the flash memory and gateelectrodes of the logic system NMOS and PMOS. The second conductivelayer has a laminated structure of polycrystalline silicon film, highmelting point metal silicide film and silicon oxide film which arelaminated one after another in this order from the bottom. Used as thepolycrystalline silicon film is a film having a sheet resistance ρs=60to 100 Ω/□ formed by diffusing, through thermal diffusion at 900° C. orless, phosphorous in polycrystalline silicon of a film thickness of 100to 200 nm deposited at about 640° C. The high melting point metalsilicide film is a WSix film (x=2.5 to 3.0) formed through CVD processor sputtering process to have a film thickness of 100 to 150 nm and asheet resistance ρs=2 to 15 Ω/□ after heat treatment. The silicon oxidefilm is formed through CVD process to have a thickness of 100 to 150 nm.This silicon oxide film is a protective film for the polycrystallinesilicon film and high melting point metal silicide film serving as anactual control gate electrode or a gate electrode and protects highmelting point metal from damage such as ion implantation or dry etching.

[0313] (4) The control gate electrode 11, inter-layer insulating film 9and floating gate electrode 8 of the flash memory are formed inself-align fashion through dry etching using a mask such as photoresist.

[0314] (5) The tunnel insulating film 7 suffering from damage throughdry etching in the above (4) is removed through wet etching using thefirst and second conductive layers 8 and 11 as a mask to expose thesurface of P type semiconductor substrate 1 at regions for formation ofsource and drain of the flash memory.

[0315] (6) Then an insulating film 12 is formed over the entire surface.The insulating film 12 is a protective film which is a silicon oxidefilm formed to have a thickness of 10 to 20 nm through CVD process.

[0316] (7) An N type semiconductor region 13 and a P type semiconductorlayer 14 are formed at source and drain regions of the flash memory byusing the second conductive layer 11 as a mask. Here, the N typesemiconductor region 13 is formed by injecting arsenic by about 1×10¹⁵cm⁻² at 50 to 80 keV accelerating energy through ion implantationprocess. The P type semiconductor layer 14 is formed by injecting boronby 1×10¹³ to 1×10¹⁴ cm⁻² at 20 to 60 kev accelerating energy through ionimplantation process.

[0317] (G) Process Shown in FIG. 58G

[0318] (1) Gate electrodes of the logic system NMOS and PMOS are formedthrough dry etching using a mask such as photoresist. During theetching, the flash memory region, which is covered with the mask, is notetched. The second conductive layers 11 at areas unnecessary forformation of the high breakdown voltage NMOS and PMOS and at the areafor formation of the Zener diode are removed.

[0319] (2) After the mask such as photoresist is removed, the highmelting point metal silicide of the second conductive layer 11 isrendered to have a low resistance (sheet resistance ρs=2 to 15 Ω/□)through a heat treatment at about 900 to 950° C.

[0320] (3) Subsequently, an N type semiconductor region 15 is formed atthe source region of the flash memory by using a mask such asphotoresist. The N type semiconductor region is formed by injectingphosphorous by about 5×10 cm⁻² at 50 to 80 kev accelerating energythrough ion implantation.

[0321] (4) Then, the N type semiconductor region 15 is thermallydiffused through a heat treatment conducted at about 950° C. for about30 minutes to 2 hours to cover the source region of the P typesemiconductor layer 14. Thus, the drain region has a two-layer structureof the N type semiconductor region 13 and the P type semiconductor layer14 for improving threshold control and write efficiency. The sourceregion has a two-layer structure of the N type semiconductor region 13based on arsenic and the N type semiconductor region 15 based onphosphorous for improving source breakdown voltage upon erase. Whensector erase is used for erasing wherein a control gate electrode (wordline) 11 of the flash memory is applied with a negative bias relative tothe P type semiconductor substrate 1 and erase is effected over theentire channel region under the floating gate electrode 8, the formationof the N type semiconductor region 15 on the side of source is unneeded.

[0322] (5) An N type semiconductor region 16 is formed by injectingphosphorous by 2 to 4×10¹³ cm⁻² at 50 kev accelerating energy throughion implantation process using a mask such as photoresist.

[0323] (6) A P type semiconductor region 17 is formed over the entiresurface by injecting boron by 1 to 2×10 ¹³ cm⁻² through ion implantationprocess. Boron is also injected into the NMOS region but this region hashigh concentration of phosphorous and therefore is allowed tosubstantially act as an N type semiconductor.

[0324] (H) Process Shown in FIG. 58H

[0325] (1) After a silicon oxide film is formed over the entire surfacethrough CVD process, a side wall 18 is formed through dry etching.

[0326] (2) Through ion implantation process using a mask such asphotoresist, an N type semiconductor region is formed by injectingarsenic by 1 to 5×10¹⁵ cm⁻² at 60 kev accelerating energy and a P typesemiconductor region 20 is formed by injecting boron by 1 to 2×10¹⁵ cm⁻²at 15 kev accelerating energy. A Zener diode is formed of the N typesemiconductor region 19 and P type semiconductor region 20, having aZener voltage of 3 to 4V.

[0327] (I) Process Shown in FIG. 58I

[0328] (1) An insulating film 21 is formed. The insulating film 21 isformed of a silicon oxide film of about 150 nm film thickness and a BPSGfilm of 400 to 500 nm film thickness which are prepared through CVDprocess.

[0329] (2) After a contact hole 22 is formed, a first wiring layer 23 isformed. The first wiring layer 23 is formed of a laminated film of highmelting point metal silicide and aluminum. The first wiring layer 23 isalso used as a data line and a source line of the flash memory.

[0330] (3) An insulating film 24 is formed on the first wiring layer 23.The insulating film 24 is a laminated film of a silicon oxidefilm/spin-on-glass film prepared through plasma CVD process and asilicon oxide film prepared through plasma CVD process.

[0331] (4) After a through hole 25 is formed, a second wiring layer 26is formed. The second wiring layer 26 has the same film structure as thefirst wiring layer 23. The second wiring layer 26 is used for shuntingthe second conductive layers 11 serving as word lines of the flashmemory.

[0332] (5) A final passivation film 27 is formed to end in completion.The final passivation film 27 is a laminated film of a silicon oxidefilm prepared through CVD process or plasma CVD process and a siliconnitride film prepared through plasma CVD process.

[0333] [22] A Semiconductor Substrate/well Structure Meeting SectorErase

[0334] Technical consideration to be paid to erasing the flash memory isvoltage conditions as shown in FIG. 59. If, in the case of employment ofsector erase (the control electrode is applied with a negative biasrelative to the semiconductor substrate), a circuit for generation ofthe negative bias is complicated, the control gate electrode=GND and thesubstrate=positive bias are set up to thereby perform substantialnegative bias erase. In this case, a portion of substrate at an area forformation of the flash memory needs to be separated. A semiconductorsubstrate/well structure for this purpose will be described withreference to FIGS. 60 to 62.

[0335] (A) Structure Shown in FIG. 60

[0336] N type wells 2 and P type wells 3 are formed in a major surfaceof an N type semiconductor substrate 101 to accomplish separation. Tothis end, as shown in FIG. 67, the N type semiconductor substrate 101 isused in place of the P type semiconductor substrate 1.

[0337] (B) Structure Shown in FIG. 62

[0338] A double well structure (P type well 3/N type well 2/P typesemiconductor substrate 1) is used for separation. In this case,

[0339] (1) An N type well 2 is formed in a major surface of the P typesemiconductor substrate 1. At that time, an N type well 2 is also formedat an area for formation of the flash memory and besides,

[0340] (2) The P type well 3 is made to be more shallow than the N typewell 2.

[0341] (C) Structure Shown in FIG. 62

[0342] A double well structure (P type well 3/N type well 102/P typesemiconductor substrate 1) is used for separation. In this case,

[0343] (1) A deep N type well 102 is formed in a major surface of a Ptype semiconductor substrate 1 at an area for formation of the flashmemory, and

[0344] (2) The production is subsequently carried out in the same manneras in the case of FIG. 60.

[0345] The following operation and effect can be brought about by theforegoing embodiments.

[0346] (1) When information is initially written in the flash memoryFMRY built in the microcomputer MCU before the microcomputer MCU ismounted on a given system, the information can be written efficientlyunder the control of an external write device such as PROM writer PRW bydesignating the second operation mode. Also, by designating the firstoperation mode to the microcomputer MCU, information stored in the flashmemory FMRY can be rewritten with the microcomputer MCU mounted on thesystem. At that time, the rewrite time can be reduced by thesimultaneous erase function.

[0347] (2) By providing a plurality of memory blocks (LMB, SMB) havingmutually different storage capacities, each as a simultaneously erasableunit, in the flash memory FMRY, programs, data tables and control data,for example, can be held in each memory block in accordance its storagecapacity. More particularly, data of a relatively large amount ofinformation can be written in a memory block having a relatively largestorage capacity and data of a relatively small amount of informationcan be written in a memory block having a relatively small storagecapacity. In other words, a memory block of a storage capacity meetingan amount of information to be stored can be utilized. Accordingly, suchinconvenience in that a memory, although sufficing for a program, cannothe easily used as a data area because of an excessively large erase unitcan be prevented. Further, even when a given memory block is erasedsimultaneously for rewrite of part of information held in the flashmemory, such wastefulness that information is erased together with agroup of information which need not substantially be rewritten andthereafter the information group is again written can be prevented asfar as possible.

[0348] (3) Of the plurality of memory blocks, a memory block having astorage capacity which is set to be smaller than that of a built-in RAMcan be provided so that the built-in RAM may be used as a working areaor a data buffer area for rewrite of that memory block.

[0349] (4) When in the above (3) the flash memory is rewritten with themicrocomputer mounted, information in the memory block to be rewrittenis transferred to the built-in RAM, only partial information to berewritten is received from the outside and rewritten on the RAM and thenrewrite of the flash memory is carried out, thereby ensuring thatinformation held internally in advance of rewrite and not required to berewritten need not be transferred additionally from the outside andwastefulness of information transfer for partial rewrite of the memoryblock can be eliminated.

[0350] (5) In the flash memory, the simultaneous erase time is not soshort even for a small memory block and consequently the flash memoryper se cannot be rewritten on real time base in synchronism with acontrol operation by the microcomputer MCU. But by utilizing thebuilt-in RAM as a working area or a data buffer area for rewrite of amemory block, the same data as that rewritten on real time base caneventually be obtained in the memory block.

[0351] (6) By incorporating in the flash memory FMRY a register MBREG inwhich information for designating a memory block to be erasedsimultaneously is held rewritably, a memory block to be erasedsimultaneously can be designated internally and externally of themicrocomputer MCU (built-in central processing unit, external PROMwriter) with ease in accordance with the same procedure.

[0352] (7) Thanks to the aforementioned operation and effect, ease ofuse of the flash memory FMRY built in the microcomputer MCU can beimproved.

[0353] (8) As shown in FIG. 24, one bit of input/output data correspondsto one memory mat. By employing this one memory mat per one I/Ostructure, a common data line CD can be separated at each memory mat andneed not extend over a long distance which passes through all of thememory mats, so that parasitic capacitance associated with the commondata line can be reduced to contribute to speed-up of access and a lowvoltage operation.

[0354] (9) By defining memory blocks each in a unit of word line, theminimum memory block in the whole of memory array ARY has a storagecapacity corresponding to that of one word line. This holds trueregardless of the number of parallel input/output bits of the flashmemory. Accordingly, by defining memory blocks each in a unit of wordline, the storage capacity of the minimum memory block can be made to besmall more easily and especially, in the case of a memory which is builtin a microcomputer and in which input/output of data is effected in aunit of byte or word, the minimum size of memory block can be reduceddrastically. Through this, ease of use of the flash memory built in themicrocomputer can further be improved, thus contributing to improvementin efficiency of rewrite of small scale data in a unit of memory block.

[0355] (10) As shown in FIG. 26, when voltage Vddi such as 3.5V isapplied to the source of a memory cell in an unselected block forwriting to raise potential on the side of source, data line disturbancewhich decreases the threshold of a memory cell transistor can beprevented.

[0356] (11) For prevention of the data line disturbance, it is effectiveto minimize the data line disturbance time. In this case, the data linedisturbance time affecting a small memory block owing to writeconcomitant with rewrite of a memory block of a large memory capacity isrelatively longer as compared to the converse case. By taking advantageof this fact, with respect to the intervening transfer gate circuit TGC,memory blocks MBb on the side of the Y selection circuit YSEL are formedof large memory blocks having relatively large storage capacities andmemory blocks MBa on the opposite side are formed of small memory blockshaving relatively small storage capacities. Through this, the data linedisturbance time affecting memory cells of a memory block MBb owing towrite of a memory block MBa can be far more decreased in the case wherethe memory blocks MBa are small memory blocks and the memory blocks MBbare large memory blocks than in the case where the memory blocks MBa areotherwise large memory blocks and the memory blocks MBb are otherwisesmall memory blocks. In this manner, prevention of erroneous operationsdue to data line disturbance can further be perfected.

[0357] (12) By arranging dummy word lines DWA and DWB and dummy cellsDC0 to DC6 at opposing ends of memory blocks which are separated by thetransfer gate circuit TGC, irregularity in dimensions of word lines andcontrol gates near the transfer gate circuit TGC can be reduced.

[0358] The invention achieved by the present inventors has beendescribed specifically on the basis of embodiments but the presentinvention is not limited thereto and may obviously be changed in variousways without departing from the gist of the invention.

[0359] For example, the peripheral circuits incorporated in themicrocomputer are not limited to those in the forgoing embodiments butmay be changed suitably. Memory cell transistors of the flash memory arenot limited to MOS transistors of stacked gate structure in theforegoing embodiments and memory cell transistors of FLOTOX type using atunnel phenomenon also in a write operation can also be used. In theforegoing embodiments, controlling of both of erase and write of theflash memory is realized by way of software means as shown in FIGS. 22and 23 but the invention is not limited thereto and for example,simultaneous erase requiring relatively much time may be controlled bydedicated hardware built in the flash memory. For example, the dedicatedhardware includes control logic for controlling setting and clear of Ebit and EV bit and for performing verify of the erase state. Theincorporation of the control logic for simultaneous erase into the flashmemory can, on the one hand, improve ease of use by the user in thatsoftware load concerning simultaneous erase can be mitigated but on theother hand increases the area of the control logic. As for the contentsof items [1] to [7], in addition to a memory block in which the unit ofsimultaneous erase is effected in a unit of common source line, a memoryblock may be used in which the word line is used as a common line inerase. Which one of the memory blocks is to be selected can bedetermined by taking into consideration circumstances of what polarityof erase voltage is used or which one of the number of memory cellsconnected to a single word line and the number of memory cells connectedto a single data line is smaller when the storage capacity ofsimultaneous erase unit is to be minimized. The size of memory block isnot limited to the fixed size in the foregoing embodiments. For example,the size can be varied in accordance with setting of the controlregister or designation by the mode signal. For example, whensimultaneous erase voltage is applied to a word line defined as aminimum unit, the operation of the driver for driving the word line withthe erase voltage can be selected in accordance with the setting of thecontrol register or the designation by the mode signal. Further, as forthe division of memory blocks, the whole may be divided into a pluralityof large blocks LMB0 to LMB7 and each large block may be divided into aplurality of small blocks SMB0 to SMB7 as shown in FIG. 24, wherebysimultaneous erase can be done in a unit of large block or in a unit ofsmall block. In a memory cell transistor of the flash memory,its sourceand drain are relatively defined in accordance with voltages appliedthereto.

[0360] The present invention can be applied widely to a flash memory inwhich write can be done by effecting simultaneous erase in a unit of atleast memory block and to a microcomputer conditioned by comprising, ona single semiconductor chip, a central processing unit and a flashmemory which is electrically rewritable.

What we claim is:
 1. A method for manufacturing a printed board on whicha semiconductor device is mounted, the method comprising: preparing aresin sealed semiconductor device which includes a semiconductorsubstrate on which a CPU, a serial communication interface and a flashmemory are formed, wherein the semiconductor device has (i) a first modefor writing data supplied from outside of the semiconductor substrateinto the flash memory by a PROM writer, and (ii) a second mode forwriting data supplied from outside of the semiconductor substrate to theflash memory by executing a write control program with the CPU; in thefirst mode, writing the control program to the flash memory by the RPROMwriter; mounting the semiconductor device on the printed board; andafter mounting, in the second mode, rewriting the flash memory withexternally inputted data via the serial communication interface.
 2. Amethod according to claim 1, wherein the rewriting includes: erasingdata stored in the flash memory; writing data into the flash memory; andverifying data in the flash memory.
 3. A method according to claim 1,wherein the resin sealed semiconductor device includes a flat packagehaving external terminals in four directions.
 4. A method according toclaim 3, wherein the distance between adjacent external terminals is 0.5mm or less.
 5. A method according to claim 1, wherein the flash memoryhas a plurality of memory blocks, each block being defined as asimultaneously erasable unit, and wherein ones of the plurality ofmemory blocks are rewritten in the second mode.
 6. A method according toclaim 5, wherein the rewriting includes: erasing data stored in theflash memory; writing data into the flash memory; and verifying data inthe flash memory.
 7. A method according to claim 6, wherein the resinsealed semiconductor device includes a flat package having externalterminals in four directions.
 8. A method according to claim 7, whereinthe distance between adjacent external terminals is 0.5 mm or less.
 9. Amethod according to claim 5, wherein the resin sealed semiconductordevice includes a flat package having external terminals in fourdirections.
 10. A method according to claim 9, wherein the distancebetween adjacent external terminals is 0.5 mm or less.